<form>
    <input type="number" id="num1">
    <input type="number" id="num2">
    <input type="button" value="oi" onclick="calpp()">
</form>
<script>
    var x = parseInt(document.getElementById('num1').value, 10);
    var y = parseInt(document.getElementById('num2').value, 10);

    function calpp(){
        alert(x+y);
    }
</script>