#include <fstream>
#define NMax 10000

int lg, max, lit[27];
char a[NMax];

void citire();
void bsort();
int main()
{
	citire();
	bsort();
	return 0;
}
void bsort()
{
	int i, j;

	for (i=0; i<lg; i++)
	{
		lit[a[i]-'a']++;
		for (j=a[i]-'a'+1; j<26; j++)
			max += lit[j];
	}
		
	std::ofstream g("litere.out");
	g << max << '\n';
}
void citire()
{
	std::ifstream f("litere.in");
	f >> lg >> a ;
	f.close();
}