#include <cstdio>

const int N = 10000;
const int L = 32;

int n;
char s[N];
int f[L];

int main() {
	freopen("litere.in","rt",stdin);
	freopen("litere.out","wt",stdout);
	scanf("%d",&n);
	scanf("%s",s);
	int r = 0;
	for (int i = 0; i < n; ++i) {
		++f[s[i]-'a'];
		for (int j = s[i]-'a'+1; j < L; ++j) {
			r += f[j];
		}
	}
	printf("%d\n",r);
	return 0;
}