function Main(input) {
  input = input.trim().split("\n").map(function(x) { return x.split(" ")});
  var S = input[0][0];
  var T = input[1][0];
  var vec_S = [];
  var vec_T = [];
  for (var i = 0; i < S.length; i ++){
    vec_S.push(S.substr(i,1));
    vec_T.push(T.substr(i,1));
  }
  var ans = 0;

  for (var i = 0; i < vec_S.length; i ++){
    if (vec_S[i] !== vec_T[i]) ans ++;
  }

  console.log(ans);
}
 
Main(require("fs").readFileSync("/dev/stdin", "utf8"));
