#include <cstdio>

const int S = 30;
const int N = 10001;

short frecv[S];

char s[N];

void citire()
{
	int n;
	scanf("%d\n",&n);
	scanf("%s",s);
}

int pasi(char c)
{
	int r = 0;
	for (char ch = c + 1; ch <= 'z'; ++ch)
		r += frecv[ch - 'a'];
	return r;
}

void calculare_si_afisare()
{
	int r=0;
	for (int i = 0; s[i]!=0; ++i)
	{
		r += pasi(s[i]);
		++frecv[s[i] - 'a'];
	}
	printf("%d",r);
}

int main()
{
	freopen("litere.in","r",stdin);
	freopen("litere.out","w",stdout);
	citire();
	calculare_si_afisare();
	return 0;
}