"use strict";

function main(input) {
  input = input.trim().split("\n");
  const line_one_arr = input[0].split("");
  const line_two_arr = input[1].split("");
  let ans = 0;
  for (let i in line_one_arr) {
    if (line_one_arr[i] !== line_two_arr[i]) {
      ans++;
    }
  }

  console.log(ans);
}

function numberSort(array) {
  array.sort(f);
  return array;
}

var f = function (a, b) {
  return a - b;
};

main(require("fs").readFileSync("/dev/stdin", "utf-8"));
