using namespace std;
#include <cstdio>
#include <cstring>
#include <cassert>
int n;
char s[10005];
int f[26];


int main(){
	freopen("litere.in","r",stdin);
	scanf("%d",&n);
	fgets(s, 100005, stdin);
	fgets(s, 100005, stdin);
	s[strlen(s)-1]=0;
	assert((int)strlen(s)==n);
	int rez=0;
	for(int i=0;s[i];++i){
		for(int j=s[i]-'a'+1; j<26;++j)
			if(f[j])
				rez+=f[j];
		f[s[i]-'a']++;
	}
	freopen("litere.out","w",stdout);
	printf("%d\n",rez);
	return 0;
}