body {
  font-family: 'Segoe UI', sans-serif;
  background: #f4f6fa;
  color: #333;
  padding: 30px;
  max-width: 800px;
  margin: auto;
}

h1, h2 {
  color: #2c3e50;
}
a {
  color: #1e90ff;
  text-decoration: none;
}
nav {
  margin-bottom: 20px;
}
nav a {
  margin-right: 15px;
}
.problem-box {
  background: white;
  padding: 20px;
  border-radius: 8px;
  box-shadow: 0 2px 8px rgba(0,0,0,0.1);
  margin-top: 20px;
  text-align: center;
}
button {
  background-color: #1e90ff;
  border: none;
  padding: 10px 15px;
  color: white;
  font-size: 1rem;
  border-radius: 5px;
  cursor: pointer;
  margin-top: 10px;
}
button:hover {
  background-color: #006dcc;
}
input[type=number] {
  font-size: 1.1rem;
  padding: 8px;
  width: 100px;
  margin-top: 10px;
}
.feedback {
  margin-top: 15px;
  font-weight: bold;
  font-size: 1.2rem;
}