Terminator004,
<!Doctype html>
<html>
<head>
<title>Taskai</title>
<meta charset="utf-8">
<script>
window.addEventListener('DOMContentLoaded', function() {
function taskai(){
var a = +prompt("iveskite skaiciu");
var b = +elem.innerHTML + +a;
localStorage.setItem('taskai', b);
elem.innerHTML = b;
}
var elem = document.getElementById("taskas");
elem.innerHTML = localStorage.getItem('taskai') || '';
document.getElementById("mygtukas").addEventListener("click",taskai)
});
</script>
</head>
<body>
<input type="button" id="mygtukas" value="Taskai" ></input>
<div>
Taskai:
<span id="taskas"></span>
</div>
</body>
</html>