/* ===== 1) RESET + FONT ===== */
* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
  font-family: "Segoe UI", Tahoma, Geneva, Verdana, sans-serif;
}

/* ===== 2) NỀN TRANG + BỐ CỤC ===== */
body {
  background: linear-gradient(135deg, #6dd5ed, #2193b0);
  min-height: 100vh;
  display: flex;
  justify-content: center;
  align-items: flex-start;
  padding: 30px;
  color: #333;
}

/* ===== 3) THẺ CHÍNH ===== */
.baiTap {
  background: #fff;
  border-radius: 16px;
  box-shadow: 0 8px 25px rgba(0, 0, 0, 0.15);
  padding: 45px;
  width: 100%;
  max-width: 1500px;
  animation: fadeIn 0.5s ease-in-out;
}

/* ===== 4) TIÊU ĐỀ ===== */
h1,
h2,
h3 {
  margin-bottom: 15px;
  font-weight: 900;
  color: #222;
}

h1 {
  color: #0b7285;
  text-align: center;
  font-size: 50px;
}

h2 {
  color: #0b7285;
  margin: 20px 0;
}

/* ===== 5) INPUT NUMBER ===== */
.arrAll {
  text-align: center;
}
input[type="number"] {
  border-radius: 10px;
  padding: 10px 12px;
  border: 2px solid #07414b5d;
  outline: none;
  transition: border-color 0.3s ease;
  margin: 10px 0;
  width: 40%;
}

input[type="number"]:focus {
  border-color: #105c6b;
  border: 4px solid #19889e;
  box-shadow: 0 0 6px rgba(12, 166, 120, 0.5);
}

/* ===== 6) BUTTON ===== */
button {
  padding: 10px 18px;
  border: none;
  border-radius: 10px;
  background: #0ca678;
  color: white;
  font-weight: 600;
  cursor: pointer;
  transition: all 0.3s ease;
  margin: 5px;
}

button:hover {
  background: #099268;
  transform: translateY(-2px);
  box-shadow: 0 4px 10px rgba(0, 0, 0, 0.2);
}

/* ===== 7) MENU CHỨC NĂNG (UL/LI) ===== */
ul {
  list-style: none;
  margin: 40px 20px;
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 35px;
}

ul li {
  border: 1px solid #dee2e6;
  padding: 20px;
  border-radius: 8px;
  background: #f1f3f5;
  text-align: center;
  box-shadow: 0 4px 10px rgba(0, 0, 0, 0.2);
  cursor: pointer;
  transition: transform 0.3s ease;
  font-weight: 500;
}

ul li:hover {
  transform: scale(1.02);
  box-shadow: 0 6px 15px rgba(0, 0, 0, 0.3);
  color: #07553ee8;
  background: #bbecde7e;
}

/* ===== 8) VÙNG CHỨC NĂNG CON ===== */
.baiTap > div {
  margin-top: 20px;
  padding: 15px;
  border-radius: 12px;
  background: #f8f9fa;
  border: 1px solid #dee2e6;
  animation: fadeIn 0.5s ease;
}

/* ===== 9) KHU VỰC KẾT QUẢ ===== */
.result1,
.result2,
.result3,
.result4,
.result5,
.result6,
.result7,
.result8,
.result9,
.result10 {
  margin-top: 10px;
  color: #0b7285;
  font-weight: 600;
  min-height: 20px;
}

/* ===== 10) ANIMATION FADE-IN ===== */
@keyframes fadeIn {
  from {
    opacity: 0;
    transform: translateY(10px);
  }
  to {
    opacity: 1;
    transform: translateY(0);
  }
}

/* ===== 11) RESPONSIVE (MÀN HÌNH NHỎ) ===== */
@media (max-width: 540px) {
  ul {
    grid-template-columns: 1fr;
  }
  input[type="number"] {
    width: 100%;
  }
  .baiTap {
    padding: 20px;
  }
}

/* ===== 12) TRẠNG THÁI ACTIVE CHO MENU ===== */
ul li.active {
  background: #17cf983f;
  color: #0b7285;
  transform: none;
  box-shadow: inset 0 0 0 2px rgba(24, 100, 171, 0.15);
}