function Main(input) {
  const S = input.split("\n")[0].trim().split("");
  const T = input.split("\n")[1].trim().split("");
  let result = 0;

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

  }

  Main(require("fs").readFileSync("/dev/stdin", "utf8"));