#include<iostream>
using namespace std;

int N, nr_inv;
char x;
char litere[200];

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;
		for(int j = 122; j > x; --j)
			nr_inv += litere[j];
		litere[x]++;
	}

	cout << nr_inv << endl;

	return 0;
}