#include <iostream>
#include <cstring>
#include <algorithm>
#include <fstream>

using namespace std;

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

int n, rezultat, aparitii[300], mutari;
char p;

int main() {

	fin >> n;
	while (n--) {
		fin >> p;
		aparitii[p]++;
		mutari = 0;
		for (char i = p + 1; i <= 'z'; i++)
			mutari += aparitii[i];

		rezultat += mutari;
	}

	fout << rezultat << "\n";

	return 0;
}