:root {
    --font-primary: 'Roboto Mono', monospace;
    --color-bg: #f8f9fa;
    --color-card: #ffffff;
    --color-border: #e1e4e8;
    --color-text: #24292e;
    --color-text-secondary: #586069;
    --color-primary: #2563eb;
    --color-primary-hover: #1d4ed8;
    --color-success: #059669;
    --color-success-bg: #ecfdf5;
    --color-error: #dc2626;
    --color-error-bg: #fef2f2;
    --shadow-sm: 0 1px 2px rgba(0, 0, 0, 0.05);
    --shadow-md: 0 4px 6px -1px rgba(0, 0, 0, 0.1), 0 2px 4px -1px rgba(0, 0, 0, 0.06);
    --radius: 8px;
}

* {
    box-sizing: border-box;
}

body {
    font-family: var(--font-primary);
    margin: 0;
    padding: 0;
    font-size: 14px;
    background-color: var(--color-bg);
    color: var(--color-text);
    line-height: 1.6;
}

/* Navigation */
nav {
    display: flex;
    max-width: 1400px;
    margin: 0 auto;
    align-items: center;
    justify-content: space-between;
    padding: 15px 40px;
    background-color: var(--color-bg);
}

nav img[alt="ProblemMaster Logo"] {
    height: 40px;
}

nav a {
    text-decoration: none;
    font-weight: bold;
    color: var(--color-text);
}

.signin-container a {
    margin-left: 0;
}

nav a:hover {
    text-decoration: underline;
}

.signin-container {
    display: flex;
    align-items: center;
    gap: 8px;
}

.signin-container img {
    height: 20px;
    width: auto;
}

/* Main Content */
main {
    display: flex;
    justify-content: space-between;
    gap: 40px;
    padding: 20px 40px 60px;
    max-width: 1400px;
    margin: 0 auto;
}

.left-column {
    flex: 1;
    max-width: 50%;
}

.right-column {
    flex: 1;
    max-width: 50%;
}

/* Problem Header */
.problem-header {
    display: flex;
    align-items: center;
    gap: 16px;
    margin-bottom: 20px;
    padding-bottom: 16px;
    border-bottom: 1px solid var(--color-border);
}

.problem-header h1 {
    font-size: 20px;
    font-weight: 600;
    margin: 0;
}

.difficulty-stars {
    height: 14px;
}

/* Problem Card */
.problem-card {
    padding: 28px;
}

.problem-card .topic-bar {
    margin-top: 20px;
    padding-top: 20px;
    padding-bottom: 0;
    border-top: 1px solid var(--color-border);
}

/* Cards */
.card {
    background: var(--color-card);
    border: 1px solid var(--color-border);
    border-radius: var(--radius);
    padding: 24px;
    margin-bottom: 16px;
    box-shadow: var(--shadow-sm);
}

.card h2 {
    font-size: 14px;
    font-weight: 600;
    margin: 0 0 16px 0;
    color: var(--color-text);
}

/* Problem Card */
.problem-card {
    padding: 28px;
}

.problem-description {
    font-size: 14px;
    line-height: 1.7;
}

.problem-description p {
    margin: 0 0 12px 0;
}

.problem-description strong {
    font-weight: 600;
}

.problem-image {
    display: block;
    margin: 24px 0;
    max-width: 100%;
}

/* Answer Card */
.answer-card h2 {
    margin-bottom: 12px;
}

.answer-row {
    display: flex;
    gap: 12px;
    align-items: center;
}

.answer-input {
    flex: 0 0 200px;
    height: 40px;
    padding: 0 12px;
    border: 2px solid var(--color-border);
    border-radius: 6px;
    font-family: var(--font-primary);
    font-size: 14px;
    transition: border-color 0.2s;
}

.answer-input:focus {
    outline: none;
    border-color: var(--color-primary);
}

.check-btn {
    height: 40px;
    padding: 0 20px;
    background: var(--color-primary);
    color: white;
    border: none;
    border-radius: 6px;
    font-family: var(--font-primary);
    font-size: 14px;
    font-weight: 500;
    cursor: pointer;
    transition: background-color 0.2s;
}

.check-btn:hover {
    background: var(--color-primary-hover);
}

/* Feedback Messages */
.feedback-message {
    margin-top: 12px;
    padding: 10px 14px;
    border-radius: 6px;
    font-size: 13px;
    display: none;
}

.feedback-message:not(:empty) {
    display: block;
}

.feedback-message.success {
    background: var(--color-success-bg);
    color: var(--color-success);
    border: 1px solid var(--color-success);
}

.feedback-message.error {
    background: var(--color-error-bg);
    color: var(--color-error);
    border: 1px solid var(--color-error);
}

/* Collapsible Cards */
.collapsible-card {
    padding: 0;
}

.card-header {
    display: flex;
    justify-content: space-between;
    align-items: center;
    padding: 16px 24px;
    cursor: pointer;
    user-select: none;
    transition: background-color 0.2s;
}

.card-header:hover {
    background: #f9fafb;
}

.card-header h2 {
    margin: 0;
    font-size: 14px;
}

.toggle-icon {
    font-size: 20px;
    font-weight: 300;
    color: var(--color-text-secondary);
    width: 24px;
    text-align: center;
}

.card-content {
    padding: 0 24px 24px;
    overflow: hidden;
    transition: max-height 0.3s ease-out, padding 0.3s ease-out;
}

.card-content.collapsed {
    max-height: 0;
    padding-top: 0;
    padding-bottom: 0;
}

/* Hints Ladder */
.hints-ladder {
    display: flex;
    flex-direction: column;
    gap: 12px;
}

.hint-item {
    border: 1px solid var(--color-border);
    border-radius: 6px;
    padding: 16px;
    transition: all 0.2s;
}

.hint-item.locked .hint-body {
    filter: blur(4px);
    pointer-events: none;
    user-select: none;
}

.hint-item.solved {
    border-color: var(--color-success);
    background: var(--color-success-bg);
}

.hint-header {
    display: flex;
    justify-content: space-between;
    align-items: center;
    margin-bottom: 8px;
}

.hint-number {
    font-size: 12px;
    font-weight: 600;
    color: var(--color-text-secondary);
    text-transform: uppercase;
    letter-spacing: 0.5px;
}

.lock-icon {
    font-size: 14px;
    cursor: pointer;
    padding: 4px;
}

.lock-icon:hover {
    transform: scale(1.2);
}

.hint-text {
    margin: 0;
    font-size: 13px;
    color: var(--color-text);
}

.subproblem-answer {
    display: flex;
    gap: 8px;
    margin-top: 12px;
}

.hint-answer-input {
    flex: 0 0 120px;
    height: 32px;
    padding: 0 10px;
    border: 1px solid var(--color-border);
    border-radius: 4px;
    font-family: var(--font-primary);
    font-size: 13px;
}

.hint-answer-input:focus {
    outline: none;
    border-color: var(--color-primary);
}

.hint-check-btn {
    height: 32px;
    padding: 0 12px;
    background: transparent;
    color: var(--color-primary);
    border: 1px solid var(--color-primary);
    border-radius: 4px;
    font-family: var(--font-primary);
    font-size: 12px;
    font-weight: 500;
    cursor: pointer;
    transition: all 0.2s;
}

.hint-check-btn:hover {
    background: var(--color-primary);
    color: white;
}

.hint-feedback {
    margin-top: 8px;
    font-size: 12px;
}

.hint-feedback.success {
    color: var(--color-success);
}

.hint-feedback.error {
    color: var(--color-error);
}

/* Reasoning Section */
.solution-input {
    display: block;
    width: 100%;
    height: 150px;
    padding: 12px;
    border: 1px solid var(--color-border);
    border-radius: 6px;
    font-family: var(--font-primary);
    font-size: 13px;
    resize: vertical;
    line-height: 1.6;
}

.solution-input:focus {
    outline: none;
    border-color: var(--color-primary);
}

.solution-input::placeholder {
    color: var(--color-text-secondary);
}

.feedback-btn {
    margin-top: 12px;
    height: 36px;
    padding: 0 16px;
    background: transparent;
    color: var(--color-text);
    border: 1px solid var(--color-border);
    border-radius: 6px;
    font-family: var(--font-primary);
    font-size: 13px;
    font-weight: 500;
    cursor: pointer;
    transition: all 0.2s;
}

.feedback-btn:hover {
    border-color: var(--color-text);
    background: var(--color-text);
    color: white;
}

/* Topics Bar */
.topic-bar {
    display: flex;
    align-items: center;
    gap: 10px;
    padding: 20px 0;
    font-size: 13px;
    color: var(--color-text-secondary);
}

.topic-label {
    font-weight: 500;
}

.topic-tag {
    padding: 4px 10px;
    background: #f3f4f6;
    border-radius: 4px;
    font-size: 12px;
    color: var(--color-text);
}

/* Mobile Responsiveness */
@media (max-width: 1000px) {
    main {
        flex-direction: column;
        padding: 16px 20px 40px;
    }

    .left-column,
    .right-column {
        max-width: 100%;
    }

    .problem-header {
        flex-direction: column;
        align-items: flex-start;
        gap: 8px;
    }

    .problem-header h1 {
        font-size: 20px;
    }

    .card {
        padding: 20px;
    }

    .answer-row {
        flex-direction: column;
        align-items: stretch;
    }

    .answer-input {
        flex: 1;
        width: 100%;
    }

    .check-btn {
        width: 100%;
    }

    .subproblem-answer {
        flex-direction: column;
    }

    .hint-answer-input {
        flex: 1;
        width: 100%;
    }

    .hint-check-btn {
        width: 100%;
    }
}

/* Legacy support for visited links */
a:visited {
    color: inherit;
}

a {
    color: inherit;
}