#include <cstdio>
#include <fstream>

using namespace std;

int c[1<<5],n;
char s[1<<10];

void read (){
	
	ifstream in ("litere.in");
	in>>n;
	in.getline(s,1000);
	in.getline(s,1000);
	in.close ();
	
	}
	
	int solve (){
		
		int r=0;
		for(int i=1;s[i];++i){
			for(int j=s[i]-'a'+1;j<26;++j)
			if(c[j])
			r+=c[j];
			++c[s[i]-'a'];
			}
		
		return r;}
		
		void write (int x){
			
			freopen ("litere.out","w",stdout);
			printf("%d\n",x);
			
			}

int main ()
{
	
	read ();
	write (solve ());
	
	return 0;}