[Dilettante_Pro]

Будет

<div>
	<input id="inp"></input>
	<button onclick='test1();'>plus element</button>
	<button onclick='test2();'>=</button>
	<p id="tx"></p>
	<p id="tx2"></p>
</div>
<script>
var g=[];
function test1(){
	var a=document.getElementById("tx");
	var f=document.getElementById("inp").value;
	g.unshift(+f);
	a.innerHTML=g;
}
function test2(){
	var t=document.getElementById("tx2");
	var sum = g[0]+g[1];
	t.innerHTML=sum;
}
</script>