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

body {
  background: #f5f7fa;
}

nav {
  background: #fff;
  padding: 15px 20px;
  display: flex;
  justify-content: space-between;
  align-items: center;
  box-shadow: 0 2px 10px rgba(0, 0, 0, .1);
}

.back-btn {
  text-decoration: none;
  color: #28a745;
  font-weight: bold;
}

.calculator {
  max-width: 650px;
  margin: 40px auto;
  background: #fff;
  padding: 30px;
  border-radius: 10px;
  box-shadow: 0 5px 20px rgba(0, 0, 0, .08);
}

.calculator h1 {
  text-align: center;
  margin-bottom: 25px;
}

.form-group {
  margin-bottom: 20px;
}

.form-group label {
  display: block;
  margin-bottom: 8px;
}

.form-group input,
.form-group select {
  width: 100%;
  padding: 12px;
  border: 1px solid #ccc;
  border-radius: 5px;
}

button {
  width: 100%;
  padding: 12px;
  border: none;
  background: #28a745;
  color: white;
  border-radius: 5px;
  cursor: pointer;
}

.result {
  margin-top: 25px;
  background: #f8f9fa;
  padding: 20px;
  border-radius: 8px;
}

.result p {
  margin-bottom: 10px;
  font-size: 18px;
}