function main(lines) {
  const S = lines[0];
  const T = lines[1];

  const len = S.length;
  let res = 0;
  if (S !== T) {
    for (let i = 0; i < len; i++) {
      if (S[i] !== T[i]) res++;
    }
  }
  
  console.log(res);
}


main( require("fs").readFileSync("/dev/stdin", "utf8").trim().split("\n") );



function splitN(s) {
  return s.split(' ').map(Number);
}