#include <iostream>
#include <fstream>

using namespace std;

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

short frequency[30];

int main()
{
    ios_base :: sync_with_stdio(false);
    fin.tie(nullptr), fout.tie(nullptr);

    short n;
    int cnt = 0;
    char c;
    fin>>n;

    for (short i = 1; i <= n; i++) {
        fin>>c;

        for (short j = c+1; j <= 'z'; j++) cnt += frequency[j-'a'];

        frequency[c-'a']++;
    }

    fout<<cnt;
    return 0;
}