<form class="needs-validation" method="POST" action="/generate_rotp_key">
  <div class="text-center mb-4 mt-4">
    <img class="mb-4" src="<%= @qr_base64 %>" alt="" width="300" height="300">
    <h1 class="h3 mb-3 font-weight-normal">2段階認証</h1>
  </div>
  <label for="otp">ワンタイムパスワード</label>
  <div class="form-label-group">
    <input type="number" id="otp" class="form-control" name="otp" placeholder="ワンタイムパスワード" required autofocus>
  </div>
  <button id="submit" class="btn btn-lg btn-primary btn-block" type="submit">Check</button>
</form>