#include <fstream>

using namespace std;

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

int Ap[300], N, ans, mutari;

char c;

int main()
{
    fin >> N;

    while(N--) {
        fin >> c;

        Ap[c]++, mutari = 0;
        ///elmentele sunt sortate in orice moment
        ///introducem un element si realizam interschimbari pana cand vectorul este soratat
        for(char i = c + 1; i <= 'z'; i++)
            mutari += Ap[i];

        ans += mutari;
    }

    fout << ans << '\n';

    fin.close();
    fout.close();

    return 0;
}