#include <bits/stdc++.h>   using namespace std;   int main() {     string a, b;     int k = 0, l = 0, n;     cin >> a >> b;     n = a.size();     for (int i = 0; i < n; i++) {       if (a[i] ==  4  && b[i] ==  7 ) {         k++;       } else if (a[i] ==  7  && b[i] ==  4 ) {         l++;       }     }     int p = max(k, l);     cout << p << endl;     return 0;   }   