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<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
  <button id="btn">Ok</button>

<!-- The core Firebase JS SDK is always required and must be listed first -->
<script src="https://www.gstatic.com/firebasejs/7.14.0/firebase-app.js"></script>

<!-- TODO: Add SDKs for Firebase products that you want to use
     https://firebase.google.com/docs/web/setup#available-libraries -->

<script>
  // Your web app's Firebase configuration
  var firebaseConfig = {
    apiKey: "AIzaSyAElLWsc3qlpvG-Tb5rMNbfxVnKfL9y9bo",
    authDomain: "html-messenger-app.firebaseapp.com",
    databaseURL: "https://html-messenger-app.firebaseio.com",
    projectId: "html-messenger-app",
    storageBucket: "html-messenger-app.appspot.com",
    messagingSenderId: "569400380965",
    appId: "1:569400380965:web:79bf2b724db94b98e9b9cc"
  };
  // Initialize Firebase
  firebase.initializeApp(firebaseConfig);
</script>

<script src="script.js"></script>
</body>
</html>