#include <bits/stdc++.h>   using namespace std;   int m[120], n, k = 0;   int main() {     cin >> n;     char a, b;     getchar();     for (int i = 1; i < n; i++) {       a = getchar();       b = getchar();       m[a -  a ]++;       if (m[b -  A ] > 0) {         m[b -  A ]--;       } else {         k++;       }     }     cout << k << endl;     return 0;   }   