#include <iostream>
#include <fstream>

using namespace std;

int main()
{
	ifstream f("litere.in");

	int N;
	f >> N;

	char c;

	// read end of line characters ('\r' and '\n')
	do {
		c = f.peek();
		if (f.eof())
			return 1;
		if (c != '\r' || c != '\n')
			break;
	} while (true);

	long totalInversions = 0;
	long letterCount[26] = {0};

	while (N--) {
		f >> c;
		int crtLetter = c - 'a';
		for (int i = crtLetter + 1; i < 26; i++)
			totalInversions += letterCount[i];
		letterCount[crtLetter]++;
	}

	f.close();

	ofstream fout("litere.out");
	fout << totalInversions;
	fout.close();

	return 0;
}