SO-21694235

by Joe

HTML

<input name="1" id="1" value="">
<input name="2" id="2" value="">
<input name="3" id="3" value="" readonly>

JavaScript

var input = $('[name="1"]'),
    input1 = $('[name="1"]'),
    input3 = $('[name="3"]');
input.keyup(function () {
    var val1 = (isNaN(parseInt(input1.val()))) ? 0 : parseInt(input1.val());
    input3.val(val1);
});