#include <fstream>
#include <cstring>

using namespace std;

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


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 = i + 1; j <= n; j++)
		{
			if (s[j] > s[i])
			{
				k++;
				aux = s[i];
				s[i] = s[j];
				s[j] = aux;
			}
		}
	}
	g << k;
	return 0;
}