<!DOCTYPE html>
<html>
<head>
  <title>Your Application</title>
</head>
<body>
  <h2>Number Adder</h2>
  <form method="POST">
    <label for="number1">Number 1:</label>
    <input type="text" id="number1" name="number1"><br>
    <label for="number2">Number 2:</label>
    <input type="text" id="number2" name="number2"><br>
    <input type="submit" value="Calculate Sum">
  </form>
  {% if sum %}
  <p>Sum: {{ sum }}</p>
  {% endif %}
</body>
</html>