#include <bits/stdc++.h>
using namespace std;ifstream fin("litere.in");ofstream fout("litere.out");const int nmax = 10001;int  n, fr[305], sol;int main(){fin >> n;char s;for(int i=1; i <= n; i++){fin >> s;fr[s]++;for(int j = s + 1; j <= 'z'; j++)sol += fr[j];}fout << sol;return 0;}