input: Write a function to generate an email address using the provided username and domain.
  assert generate_email_address("john.doe", "example.com") == "john.doe@example.com"
  assert generate_email_address("alice", "mail.com") == "alice@mail.com"
  assert generate_email_address("user123", "domain.org") == "user123@domain.org"
output: def generate_email_address(username, domain):
      return f"{username}@{domain}"