const main = input => {
    const [ S, T ] = input.trim().split('\n');
    const length = S.length;
    let count = 0;
    for (let i = 0; i < length; i++) {
        if (S.substr(i, 1) !== T.substr(i, 1)) {
            count++;
        }
    }

    console.log(count)
};

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