Untitled

function sum() {
    var y = document.getElementById("txt1").value;
    var z = document.getElementById("txt2").value;
    var x = Number(y) + Number(z);
    document.getElementById("demo").innerHTML = "[" + y + " ] + [ " + z + " ] = " + x;
  }

<!doctype html>
<html lang="en">
  <head>
    <title>Calculator</title>
    <meta charset="utf-8">
    <style>
      html, body {
        height: 100%;
      }
      body {
        display: flex;
        align-items: center;
        justify-content: center;
        flex-direction: column;
        margin: 0;
      }
    </style>
    <script src="index2.js"></script>
  </head>

  <body>
    <input type="number" id="txt1">
    <input type="number" id="txt2">
    <button onclick="sum()">Calculate</button>
    <p id="demo"></p>
  </body>
</html>