<html>
<body>
<script>
d = document.createElement('div');
document.body.appendChild(d);
var frm = document.createElement('form');
d.appendChild(frm);
var now_input = document.createElement('input');
now_input.id = 'input';
document.body.appendChild(now_input);
var now_input = document.createElement('input');
now_input.id = 'answer';
document.body.appendChild(now_input);
var btn = document.createElement('button');
btn.appendChild(document.createTextNode('Счет'));
frm.appendChild(btn);
btn.onclick = function calculate()
{
var input = document.getElementById('input').value;
var arr = input.split('+');
if (arr.length > 1)
document.getElementById('answer').value = parseInt(arr[0]) + parseInt(arr[1]);
var arr = input.split('*');
if (arr.length > 1)
document.getElementById('answer').value = parseInt(arr[0]) * parseInt(arr[1]);
var arr = input.split('/');
if (arr.length > 1)
document.getElementById('answer').value = parseInt(arr[0]) / parseInt(arr[1]);
var arr = input.split('-');
if (arr.length > 1)
document.getElementById('answer').value = parseInt(arr[0]) - parseInt(arr[1]);
}
</script>
</body>
</html>