#include<stdio.h>

#define N 10001

#define A 27

int n,sum[A];

char v[N];

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

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