why dont this work

<html>
<head>
	<title></title>
</head>
<body>
<div style="margin-left:30%;margin-top:10%;">
<input id="calc">
<button onclick="anybutton("add")">+</button>
<button onclick="anybutton("sub")">-</button>
<button onclick="anybutton("multiply")">*</button>
<button onclick="show()">=</button>

</div>
</body>

<script type="text/javascript">
	function anybutton(opr) {
		firstnum=document.getElementById("calc").value;
		document.getElementById("calc").value=""
		method=opr;}

	function show() {

		secnum=document.getElementById("calc").value;
		if (method=="add"){document.getElementById("calc").value= Number(firstnum)+ Number(secnum);}
			else if (method=="sub"){document.getElementById("calc").value= Number(firstnum)- Number(secnum);}
		else if(method=="multiply"){document.getElementById("calc").value= Number(firstnum)* Number(secnum);}
	}


</script>
</html>