#include <bits/stdc++.h>
using namespace std;
int find(string& a, string& b) {
  int n1 = 0, n2 = 0;
  for (int i = 0; i < a.size(); i++) {
    if (a[i] > b[i]) n1++;
    if (a[i] < b[i]) n2++;
  }
  return n1 + n2 - min(n1, n2);
}
int main() {
  string a;
  string b;
  getline(cin, a);
  getline(cin, b);
  cout << find(a, b) << endl;
  return 0;
}
