function Main(input){
    const ST = input.split('\n');
    const S = ST[0];
    const T = ST[1];
    let count = 0;
    for(let i=0;i<S.length;i++){
        if(S[i] !== T[i]){
            count++;
        }
    }
    console.log(count);
}
Main(require("fs").readFileSync("/dev/stdin", "utf8"));
