parseInt in prompt in js.html

<html>

<head>
	<title>Sum with parseInt</title>
</head>

<body>
	<script>
		var imFirstNum;
		var imSecondNum;
		var sum = 0;
		a = prompt("Enter value of 1st Number:", "10");
		b = prompt("Enter value of 2nd Number", "20");

		document.write("value of 1st Number is " + imFirstNum + " value of 2nd Number is " + imSecondNum + "<br>");
		sum = parseInt(imSecondNum) + parseInt(imFirstNum);
		document.write("sum of value is= " + sum + "<br>");
		document.write("Completed");

	</script>
</body>

</html>