//#include "pch.h"
#include <fstream>
#include <string>
#include <algorithm>
#include <vector>
#include <cmath>

using namespace std;

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

const int SIGMA = 26;

int main() {

	int N;
	fin >> N;

	string s;
	fin >> s;

	vector <int> freq(SIGMA);

	int inv = 0;
	for (int idx = 0; idx < N; ++idx) {

		for (int ch = s[idx] + 1 - 'a'; ch <= 'z' - 'a'; ++ch)
			inv += freq[ch];
		++freq[s[idx] - 'a'];
	}

	fout << inv;
}