#include <fstream>
using namespace std;

int n, ap[30], sol;
char a;
int main()
{
    ifstream f("litere.in");
    ofstream g("litere.out");

    f >> n;
    for (int i = 0; i < n; ++i)
    {
        f >> a;
        ++ap[a - 'a'];
        for (int j = a - 'a' + 1; j <= 'z' - 'a'; ++j)
            sol = sol + ap[j];
    }

    g << sol;
    g.close();
    return 0;
}