#include <fstream>
using namespace std;

#define nmax 10001
int n,lit[27];
long s;
char sir[nmax];

inline void citire()
{
	ifstream in("litere.in");
	in>>n;
	in>>sir;
}

inline void afisare()
{
	ofstream out("litere.out");
	out<<s;
}

void rez()
{
	int i,j;
	
	for(i=0;i<n;i++)
	{
		lit[int(sir[i])-'a']++;
		for(j=int(sir[i])-'a'+1;j<=26;j++)
			s += lit[j];
	}

	
}

int main()
{
	citire();
	rez();
	afisare();
	return 0;
}