#include <stdio.h>
#define N 10005
char v[N];
int alph[28],n,sum;
void Solve()
{
	scanf("%d\n",&n);
	for (int i=0; i<n; i++)
		scanf("%c",&v[i]);
	for (int i=0; i<n; i++)
	{
		alph[v[i]-'a']++;
		for (int j=v[i]-'a'+1; j<='z'-'a'; j++)
			sum+=alph[j];
	}
	printf("%d\n",sum);
}
int main ()
{
	freopen("litere.in","r",stdin);
	freopen("litere.out","w",stdout);
	Solve();
}