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

int main() {

	ifstream fin("litere.in");
	ofstream fout("litere.out");
	int n;
	fin >> n;
	int f[27] = {0};
	long long schimbari = 0;

	for (int i = 0; i < n; i++) {
		char c;
		fin >> c;

		int x = c - 'a';

		for (int j = x + 1; j < 26; j++)
			schimbari += f[j];

		f[x]++;
	}

	fout << schimbari;
	return 0;
}