# include <fstream>
using namespace std;

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

int n, frec[28], i, j, nr;
char a;

int main()
{
	f >> n;
	for (i = 1; i <= n; ++i)
		{
		 f >> a;
		 frec[a-96]++;
		 for (j = (a-95); j <= n; ++j)
			 if (frec[j] != 0)
				 nr += frec[j];
		}
	g << nr;
	g.close();
	return 0;
}