"use strict";

const main = (input) => {
    const [s, t] = input.split('\n');
    const n = s.length;
    let ans = 0;
    for (let i = 0; i < n; i++){
        if (s[i] !== t[i]) ans++;
    }

    console.log(ans);
};

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