#include <bits/stdc++.h>
using namespace std;

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

int main()
{
    char c;
    int nr, contor = 0, v[100] = {0};
    fin >> nr;

    for (int i = 0; i < nr; i++)
    {
       fin >> c;
       
       int a = c - 'a';
       for (int j = a + 1; j < 26; j++) {
           contor += v[j];
       }
       
       v[a]++;
    }

    fout << contor;

    return 0;
}