* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
    font-family: sans-serif;
}

body {
    background: linear-gradient(135deg, #e0f2fe 0%, #f0f9ff 100%);
    min-height: 100vh;
}

.container {
    min-height: 100vh;
    display: flex;
    justify-content: center;
    align-items: center;
    padding: 24px;
}

.card {
    width: 100%;
    max-width: 400px; /* Caps width on big screens, full-width on mobile */
    padding: 28px 24px;
    background-color: #ffffff;
    border: 1px solid #e5e7eb;
    border-radius: 1rem;
    box-shadow: 0 10px 25px rgba(0, 0, 0, 0.08);

    display: flex;
    flex-direction: column;
    gap: 18px;
}

.title {
    font-size: 1.5rem;
    color: #0f172a;
    text-align: center;
}

.input-container {
    display: flex;
    gap: 12px;
    align-items: center;
    justify-content: center;
}

.input-container label {
    font-weight: 600;
    color: #334155;
}

#number-input {
    padding: 10px 14px;
    border: 2px solid #cbd5e1;
    border-radius: 0.6rem;
    outline: none;
    flex: 1;
    font-size: 1rem;
    transition:
        border-color 0.2s,
        box-shadow 0.2s;
}

#number-input:focus {
    border-color: #38bdf8;
    box-shadow: 0 0 0 3px rgba(56, 189, 248, 0.25);
}

input::-webkit-outer-spin-button,
input::-webkit-inner-spin-button {
    -webkit-appearance: none;
    margin: 0;
}

#check-btn {
    background-color: #22c55e;
    color: white;
    padding: 12px 18px;
    border: none;
    border-radius: 0.6rem;
    font-size: 1rem;
    font-weight: 600;
    cursor: pointer;
    transition:
        background-color 0.2s,
        transform 0.1s;
}

#check-btn:hover {
    background-color: #16a34a;
}

#check-btn:active {
    transform: scale(0.98);
}

.result {
    min-height: 1.5rem;
    text-align: center;
    font-weight: 600;
    font-size: 1.05rem;
    padding: 8px 12px;
    border-radius: 0.5rem;
    /* Default state — hidden until a result is set */
    visibility: hidden;
}

.result:not(:empty) {
    visibility: visible;
}

.result.success {
    color: #15803d;
    background-color: #dcfce7;
}

.result.error {
    color: #b91c1c;
    background-color: #fee2e2;
}

.result.info {
    color: #1e40af;
    background-color: #dbeafe;
}