#include <fstream>

using namespace std;

ifstream f("litere.in");
ofstream g("litere.out");

int n, i, nr, fr[30], j;
char c;

int main()
{
    f>>n;
    f.get();

    for (i=1;i<=n;i++)
    {
        f>>c;
        fr[c-'a'+1]++;
        for (j=c-'a'+2;j<=26;j++)
        nr=nr+fr[j];
    }



    g<<nr;

    return 0;
}