#include <fstream>
#include <cstring>

using namespace std;

char s[10100], aux;
int i, j, k, n;
int x[167];


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

	f >> n;
	for (i = 1; i <= n; i++)
		f >> s[i];
	
	for (i = 1; i <= n; i++)
	{
		for (j = s[i] + 1; j <= 'z'; j++)
			k += x[j];
		x[s[i]]++;
	}
	g << k;
	return 0;
}