function Main(input){
  input = input.split("\n");
  let s = input[0];
  let t = input[1];
  let count = 0;
  for(var i=0;i<s.length;i++){
    if(s[i] !== t[i]) count++;
  }
  console.log(count);
}

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