#include <cstdio>
#include <fstream>

using namespace std;

int c[1<<5],n,r;
char s[10001];
	
	void solve (){
		
		for(int i=0;i<n;++i){
        for(int j=s[i]-'a'+1;j<=25;++j)
                r+=c[j];
        ++c[s[i]-'a'];
		}
		
		}

int main ()
{
	
	ifstream in ("litere.in");
	in>>n;
	in.getline(s,10001);
	in.getline(s,10001);
	in.close ();
	solve ();
	freopen ("litere.out","w",stdout);
	printf("%d\n",r);
	
	return 0;}