function pad(a, b) {
  for (var i = 0; i < b.length - a.length; i++) {
    a = "0" + a;
  }
  return a;
}

function stringAdd(a, b) {
  var total = "";
  var carry = 0;
  a = pad(a, b);
  b = pad(b, a);
  for (var i = b.length - 1; i >= 0; i = i - 1) {
    var x = (parseInt(a[i]) + parseInt(b[i]) + carry);
    carry = 0;
    if (x >= 10) {
      carry = 1;
      x = x - 10;
    }
    total = x.toString() + total;
  }
  if (carry == 1) {
    total = "1" + total;
  }
  return total;
}