How do I fix this error

<script type="text/javascript">
function hi(){
	alert("ji");
	var x=document.getElementById("f").value;
	var y=document.getElementById("ff").value;
	alert("x ="+x+"y ="+y);
	var z=x+y;// here i get the output as 45, if x=4 and y=5
	alert("z = "+z);
	document.getElementById("fff").innerHTML=z;

}