#include <iostream>
#include <fstream>

using namespace std;

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

int v[30];

int main()
{
    int n, nr = 0;
    char a;
    fin >> n;
    while (n--)
    {
        fin >> a;
        int x = a - 'a' + 1;
        v[x]++;
        for (int i = x + 1; i <= 26; i++)
            nr = nr + v[i];
    }
    fout << nr << '\n';
    return 0;
}