#include<stdio.h>
#include<string.h>

int a[30];
char s[10001];
int i,n,nr;
char c;

int main()
{
	freopen("litere.in","r",stdin);
	freopen("litere.out","w",stdout);
	scanf("%d\n",&n);
	n--;
	fgets(s,10010,stdin);
	nr=0;
	a[s[n]]++;
	for(i=n-1;i>=0;i--)
	{
		for(c=s[i]-1;c>='a';c--)
			nr+=a[c];
		a[s[i]]++;
	}
	printf("%d",nr);
	return 0;
}