#include <iostream>
#include <fstream>

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;
}