#include <fstream>

using namespace std;
ifstream cin("litere.in");
ofstream cout("litere.out");

int f[30];
int main()
{
    int n, i;
    char sir[10000 + 5];
    cin >> n;
    cin.get();
    cin.getline(sir , 10000 + 1);
    int rasp = 0;
    for ( i = 0; i < n; ++i )
    {
        for ( int j = sir[i] + 1 - 'a'; j < 26; ++j )
            rasp = rasp + f[j];

        ++f[sir[i] - 'a' ];
    }


    cout << rasp << '\n';
    return 0;
}