#include<cstdio>

const int N=10005;

int n,fr[30];
char s[N];

void rez()
{
	int nr=0,x;
	for (int i=0;i<n;++i)
	{
		x=(int)s[i];
		x-=97;
		for (int j=x+1;j<26;++j)
			nr+=fr[j];
		fr[x]++;
	}
	printf("%d\n",nr);
}

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