#include<stdio.h>

int i,j,ok,n,k,nr;
char a[10003];

int main()
{
	freopen("litere.in","r",stdin);
	freopen("litere.out","w",stdout);

	scanf("%d\n",&n);

	scanf("%s",&a);

	for(i='a';i<='z';i++)
	{
		nr=0;

		for(j=0;j<n;j++)
		{
			if(i==a[j])
				k+=nr;
			if(i<a[j])
				nr++;
		}

	}

	printf("%d",k);

	return 0;
}