function Main(input) {
    tmp = input.split("\n");
    s = tmp[0];
    t = tmp[1];
    sArry = s.split('');
    tArry = t.split('');
    count = 0;

    for (let i = 0; i < sArry.length; i++) {
        if (sArry[i] != tArry[i]) {
            count += 1;
        }        
    }
    
    console.log(count);
}
Main(require("fs").readFileSync("/dev/stdin", "utf8"));