#include <iostream>
#include <fstream>

using namespace std;

ifstream in ("litere.in");
ofstream out ("litere.out");

char S[10010];
int Ap[35];

int main()
{
    int N, i, j, Ans = 0;

    in >> N;
    in >> S;

    for (i = 0; i < N; i ++){
        for (j = S[i] - 'a' + 1; j < 26; j ++)
            Ans += Ap[j];

        ++ Ap[S[i] - 'a'];
    }

    out << Ans;

    return 0;
}