"use strict";

function Main(input) {
    // 1行目がinput[0], 2行目がinput[1], …に入る
    input = input.split("\n");
    let s = input[0];
    let t = input[1];
    let count = 0;

    for (let i = 0; i < s.length; i++) {
        if (s.charAt(i) != t.charAt(i)) {
            count++;
        }
    }

    //出力
    console.log(count);
}

Main(require("fs").readFileSync("/dev/stdin", "utf8"));