#include<iostream>
using namespace std;

int N, nr_inv;
char x;
int frecvente[26];

int main()
{
	freopen("litere.in", "r", stdin);
	freopen("litere.out", "w", stdout);

	cin >> N;
	nr_inv = 0;

	for(int i = 0; i < N; ++i)
	{
		cin >> x;
		frecvente[x - 'a']++;
		x -= 'a';
		for(int j = x+1; j < 26; ++j)
			nr_inv += frecvente[j];
	}

	cout << nr_inv << endl;

	return 0;
}