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

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

int main()
{
	freopen("litere.in","r",stdin);
	freopen("litere.out","w",stdout);
	scanf("%ld\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("%ld",nr);
	fclose(stdin); fclose(stdout);
	return 0;
}