<!DOCTYPE html>
<html>
<head>
    <meta charset="utf-8">
    <title>Calculator</title>
</head>
<body>
  <header>
    <h1>ViaRézo Calculator</h1>
  </header>
  <div>
    <form action="/" method="get">
      {% if expression %}
      <input type="text" name="expression" placeholder="12 / 2 + 6 * 8" value="{{ expression }}">
      {% else %}
      <input type="text" name="expression" placeholder="12 / 2 + 6 * 8">
      {% endif %}
      <button type="submit">Calculate</button>
    </form>
    {% if result %}
    <p>Result: {{ result }}</p>
    {% endif %}
  </div>
</body>
</html