"use strict";

const main = arg => {
    const inputList = arg.trim().split("\n");
    // 回答を記述
    const S = inputList[0].split("");
    const T = inputList[1].split("");

    let sum = 0;

    for (let i = 0; i < S.length; i++) {
        if (S[i] !== T[i]) {
            sum++;
        }
    }

    console.log(sum)

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