getElement by Tagname.html

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <script type="text/javascript">

      function styleChanging()
      {
          var para = document.getElementsByTagName("p");
          para[3].style.fontSize = 25;
      }
    </script>
</head>
<body>
    <p>This is 1</p>
    <p>This is 2</p>
    <p>This is 3</p>
    <p>This is 4</p>
    <p>This is 5</p>
    <br>
    <button onclick="styleChanging()"> Click Me</button>
     
</body>
</html>