#include<cstdio>
#include<cstring>

using namespace std;

const int Sigma = 26;

const int Nmax = 10010;

int main() {
	freopen("litere.in","r",stdin);
	freopen("litere.out","w",stdout);
	
	int i, j, poz, N, cnt[Sigma+1], sol=0;
	char sir[Nmax];
	
	memset(cnt,0,sizeof(cnt));
	scanf("%d \n %s",&N,sir);
	for(i=0; i<N; ++i) {
		poz=sir[i]-'a';
		++cnt[poz];
		for(j=poz+1; j<=Sigma; ++j)
			sol+=cnt[j];
	}
	printf("%d\n",sol);
	
	return 0;
}