#include <bits/stdc++.h>   const int maxi = 2000000000;   const int maxq = 1000000000;   const double eps = 1e-10;   const double pi = acos(-1);   const double inf = 1e+18;   const int mo = 1000000007;   using namespace std;   char s[111111], s1[111111];   int main() {     gets(s);     gets(s1);     int n = strlen(s);     int m = strlen(s1);     int kol = 0;     int kol1 = 0;     int sum = 0;     int ans = 0;     for (int i = 0; i < n; i++)       if (s[i] ==  4 ) kol++;     for (int i = 0; i < n; i++)       if (s1[i] ==  4 ) kol1++;     for (int i = 0; i < n; i++) {       if (kol < kol1 && s[i] ==  7  && s1[i] ==  4 ) {         s[i] =  4 ;         ans++;         kol++;       }       if (kol > kol1 && s[i] ==  4  && s1[i] ==  7 ) {         s[i] =  7 ;         ans++;         kol--;       }     }     for (int i = 0; i < n; i++)       if (s[i] != s1[i]) sum++;     cout << ans + sum / 2 << endl;     return 0;   }   