#include <stdio.h>
#define NMax 10010

const char IN[]="litere.in",OUT[]="litere.out";

int N,Rez;
char s[NMax];
int ct[35];

int main()
{
	int i,j;
	freopen(IN,"r",stdin);
	scanf("%d%s",&N,s+1);
	fclose(stdin);

	for (i=1;i<=N;++i){

		for (j='z';j>s[i];--j)
			Rez+=ct[j-'a'];
		++ct[s[i]-'a'];
	}

	freopen(OUT,"w",stdout);
	printf("%d\n",Rez);
	fclose(stdout);

	return 0;
}