#include<cstdio>
const int maxn = 10005;
using namespace std;

char s[maxn];
int cnt[30] , i , n , ans , j;

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