body {
  font-family: Arial, sans-serif;
  background: #f0f0f0;
  height: 100vh;
  display: flex;
  justify-content: center;
  align-items: center;
  margin: 0;
}

.container {
  background: white;
  padding: 25px;
  border-radius: 12px;
  box-shadow: 0 4px 15px rgba(0,0,0,0.1);
  text-align: center;
}

.math-box {
  font-size: 28px;
  line-height: 40px;
}

.number {
  letter-spacing: 3px;
}

hr {
  margin: 15px 0;
}

input {
  width: 80%;
  padding: 8px;
  font-size: 20px;
  margin-bottom: 10px;
}

button {
  font-size: 18px;
  padding: 8px 16px;
  cursor: pointer;
}

.message {
  font-size: 22px;
  margin-top: 12px;
  height: 28px; /* prevents layout shift */
}

.back-button {
  display: inline-block;
  font-family: Roboto, Arial;
  font-size: 28px;
  position: fixed;
  left: 20px;
  top: 20px;
  padding: 5px;
  background-color: rgb(240, 240, 240);
  border-width: 2px;
  border-radius: 20px;
  border-style: solid;
  border-color: rgb(88, 87, 87);
  cursor: pointer;
}