#include <fstream>
#include <string>
using namespace std;
ifstream fin ("litere.in");
ofstream fout ("litere.out");
int N, sol, C[30];
string S;
char c;

int main()
{
    fin >> N;
    for (int i = 1; i <= N; i++)
    {
        fin >> c;
        for (int j = c - 'a' + 1; j <= 'z' - 'a' + 1; j++)
        {
            sol += C[j];
        }
        C[c - 'a'] += 1;
    }

    fout << sol << '\n';
    fout.close();
    return 0;
}