function main(arg) {
  const array = arg.trim().split("\n");
  const S = array[0];
  const T = array[1];

  if (S == T) {
    console.log(0);
    return;
  }

  let counter = 0;
  for (let i = 0; i < S.length; i++) {
    if (S[i] != T[i]) {
      counter++;
    }
  }
  console.log(counter);
}
main(require("fs").readFileSync("/dev/stdin", "utf8"));
// main("apple\napple");
//https://atcoder.jp/contests/abc167/tasks/abc167_b
