#include <cstdio>

#define file_in "litere.in"
#define file_out "litere.out"

#define Sigma 26

int N,i,j,suma=0;
char s[101010];
int v[101010];

int main(){
	
	freopen(file_in,"r",stdin);
	freopen(file_out,"w",stdout);
	
	scanf("%d\n", &N);
	gets(s);
	
	for (i=0;i<N;++i){
		 for (j=s[i]-'a'+1;j<=Sigma;++j)
               suma+=v[j];
		 v[s[i]-'a']++;
	}
	
	printf("%d\n",suma);
	
	return 0;
	
}