#include <bits/stdc++.h>   using namespace std;   ifstream Cin( input.txt );   ofstream Cout( output.txt );   int main() {     long long i, n4 = 0, n7 = 0, n;     string a, b;     cin >> a >> b;     for (i = 0; i < a.length(); i++)       if (a[i] != b[i]) {         if (a[i] ==  4 )           n4++;         else           n7++;       }     cout << n4 + n7 - min(n4, n7);   }   