#include <fstream>
#include <cstring>

using namespace std;

const int MAX = 122;
int frecv[1 + MAX];

int main()
{
    ifstream in("litere.in");
    ofstream out("litere.out");
    int n;
    int nr = 0;
    char c;

    in >> n;
    for (int i = 1; i <= n; i++)
    {
        in >> c;
        frecv[c]++;

        for (int j = c + 1; j <= 'z'; j++)
        {
            nr += frecv[j];
        }
    }

    out << nr << '\n';

    return 0;
}