#include <iostream>
#include <fstream>

using namespace std;
int N;
int ap[30];
char sir[10005];

int main()
{
    ifstream in ("litere.in");
    ofstream out ("litere.out");
    int sol = 0;
    in>>N;
    in>>sir;
    //cout<<sir;
    for(int i = 0; i < N; ++i)
    {
        int index = sir[i] - 'a';
        for(int j = index + 1; j < 26; ++j)
            sol += ap[j];
        ap[index]++;
    }
    out<<sol;
    return 0;
}