#include <iostream>
#include <fstream>

using namespace std;


int a[30], N, sol;


int main()
{
	ifstream f("litere.in");
	ofstream g("litere.out");
	
	f >> N;
	
	for (int i = 1; i <= N; ++ i)
	{
		char c;
		
		f >> c;
		
		++ a[c - 'a'];
		
		for (int j = c - 'a' + 1; j < 26; ++ j)
			sol += a[j];
	}
	
	g << sol;
	
	
	f.close();
	g.close();
	
	return 0;
}