#include <fstream>
#include <cstring>
using namespace std;

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

	int n;
	fin >> n;

	char s[10001];
	fin >> s;

	int f[26] = {0};
	int c = 0;
	for (int i = n - 1; i >= 0; i--) {
		for (int j = 0; j < s[i] - 'a'; j++)
			c += f[j];

		f[s[i] - 'a']++;
	}

	fout << c;

	return 0;
}