#include<stdio.h>

#define N 10001

#define SG 27

int n,sum[SG];

char v[N];

void calcul()
{
	long long rez=0;
	int x;
	for( int i=0 ; i<n ; ++i )
	{
		x=v[i]-'a';
		rez+=sum[x];
		for( int j=0 ; j<x ; ++j )
			++sum[j];
	}
	printf("%lld\n",rez);
}

int main()
{
	freopen("litere.in","r",stdin);
	freopen("litere.out","w",stdout);
	scanf("%d%s",&n,&v);
	calcul();
	return 0;
}