#include <fstream>

using namespace std;

ifstream fin ("litere.in");
ofstream fout("litere.out");

int n, mutari, cnt, fr[31];
char sir[10001];

int main()
{
    fin >> n;
    for(int i = 1; i <= n; i++)
        fin >> sir[i];
    for(int i = 1; i <= n; i++)
    {
        for(int j = sir[i] - 'a' + 2; j <= 26; j++)
        {
            mutari += fr[j];
        }
        fr[sir[i] - 'a' + 1]++;
    }
    fout<<mutari;
}