* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

body {
    font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
    background: linear-gradient(135deg, #06d6a0 0%, #0066cc 100%);
    min-height: 100vh;
    display: flex;
    align-items: center;
    justify-content: center;
    padding: 20px;
}

.container {
    max-width: 600px;
    width: 100%;
}

/* Header */
.header {
    text-align: center;
    color: white;
    margin-bottom: 40px;
}

.header h1 {
    font-size: 2.5rem;
    margin-bottom: 15px;
    text-shadow: 2px 2px 4px rgba(0, 0, 0, 0.3);
}

.score-display {
    font-size: 1.3rem;
    opacity: 0.95;
}

.score-display strong {
    font-size: 1.5rem;
}

/* Game Area */
.game-area {
    background: white;
    border-radius: 30px;
    padding: 50px 40px;
    box-shadow: 0 20px 60px rgba(0, 0, 0, 0.3);
    text-align: center;
}

.problem-display {
    font-size: 4rem;
    font-weight: bold;
    color: #333;
    margin-bottom: 40px;
    display: flex;
    align-items: center;
    justify-content: center;
    gap: 20px;
}

.number {
    color: #0066cc;
}

.operator {
    color: #666;
}

.equals {
    color: #666;
}

.answer-placeholder {
    color: #ccc;
}

/* Input Area */
.input-area {
    display: flex;
    gap: 15px;
    justify-content: center;
    margin-bottom: 30px;
}

#answer-input {
    flex: 1;
    max-width: 200px;
    padding: 15px 20px;
    font-size: 1.5rem;
    border: 3px solid #ddd;
    border-radius: 15px;
    text-align: center;
    transition: border-color 0.3s ease;
}

#answer-input:focus {
    outline: none;
    border-color: #0066cc;
}

#submit-btn {
    padding: 15px 40px;
    font-size: 1.2rem;
    font-weight: bold;
    color: white;
    background: linear-gradient(135deg, #00b4ff 0%, #0066cc 100%);
    border: none;
    border-radius: 50px;
    cursor: pointer;
    transition: all 0.3s ease;
    box-shadow: 0 5px 15px rgba(0, 102, 204, 0.3);
}

#submit-btn:hover {
    transform: translateY(-2px);
    box-shadow: 0 8px 20px rgba(0, 102, 204, 0.5);
}

/* Feedback */
.feedback {
    padding: 15px 25px;
    border-radius: 15px;
    font-size: 1.2rem;
    font-weight: 600;
    transition: all 0.3s ease;
}

.feedback.hidden {
    display: none;
}

.feedback.correct {
    background: linear-gradient(135deg, #e6fff0 0%, #d1ffe6 100%);
    color: #11998e;
    border: 2px solid #38ef7d;
}

.feedback.incorrect {
    background: linear-gradient(135deg, #ffe6e6 0%, #ffd1d1 100%);
    color: #ff6b6b;
    border: 2px solid #ff6b6b;
}

/* Footer */
.footer {
    text-align: center;
    margin-top: 30px;
}

.back-btn {
    display: inline-block;
    padding: 12px 30px;
    color: white;
    text-decoration: none;
    background: rgba(255, 255, 255, 0.2);
    border: 2px solid white;
    border-radius: 25px;
    font-size: 1rem;
    font-weight: 600;
    transition: all 0.3s ease;
}

.back-btn:hover {
    background: white;
    color: #0066cc;
    transform: translateY(-2px);
}

/* Mobile Responsive */
@media (max-width: 768px) {
    .header h1 {
        font-size: 2rem;
    }

    .score-display {
        font-size: 1.1rem;
    }

    .game-area {
        padding: 40px 25px;
    }

    .problem-display {
        font-size: 3rem;
        gap: 15px;
    }

    .input-area {
        flex-direction: column;
        align-items: stretch;
    }

    #answer-input {
        max-width: 100%;
    }

    #submit-btn {
        width: 100%;
    }
}