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

body {
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, sans-serif;
  min-height: 100vh;
  display: flex;
  justify-content: center;
  align-items: center;
  background-color: #f0f0f0;
}

.container {
  background: white;
  padding: 2rem;
  border-radius: 10px;
  box-shadow: 0 0 10px rgba(0,0,0,0.1);
  text-align: center;
}

h1 {
  color: #333;
  margin-bottom: 2rem;
}

.problem {
  font-size: 2rem;
  margin-bottom: 1.5rem;
}

input {
  width: 100px;
  font-size: 2rem;
  padding: 0.5rem;
  text-align: center;
  border: 2px solid #ddd;
  border-radius: 5px;
}

button {
  background: #4CAF50;
  color: white;
  border: none;
  padding: 1rem 2rem;
  font-size: 1.2rem;
  border-radius: 5px;
  cursor: pointer;
  margin: 1rem 0;
}

button:hover {
  background: #45a049;
}

#result {
  font-size: 1.2rem;
  margin: 1rem 0;
  min-height: 1.5em;
}

.stats {
  margin-top: 1.5rem;
  font-size: 1.1rem;
  display: flex;
  justify-content: space-around;
}

.correct {
  color: #4CAF50;
}

.incorrect {
  color: #f44336;
}

.timer {
  font-size: 1.2rem;
  color: #666;
  margin-bottom: 1rem;
}