#ifdef _MSC_VER
#define _CRT_SECURE_NO_WARNINGS
#endif
#include <fstream>
#include <cstring>
#include <algorithm>
#include <cmath>

using namespace std;

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

int n, rez, uz[30];

int main()
{
	char c;
	int i, j;

	fin >> n;
	fin.get();
	for (i = 1; i <= n; i++)
	{
		fin.get(c);
		for (j = c-'a'+1; j < 26; j++)
			rez += uz[j];
		uz[c - 'a']++;
	}
	fout << rez << '\n';
	return 0;
}