How to call Method that display a number's half value in javascript?

Html page:

<button onclick="myFunction()">Try it</button>

<p id="P4"></p>

Javascript page:

<script>

    Number.prototype.myMethod = function () {

        return this.valueOf() / 2;

    };

    function myFunction() {

        var n = 55;

        document.getElementById("demo").innerHTML = n.myMethod();

    }

</script>

Output:

27.5