#include <fstream>
using namespace std;
ifstream fin ("litere.in");
ofstream fout ("litere.out");
char s[10002],f[130];
int i,j,n,l;
int main () {
	fin>>n;
	fin>>s;
	for (i=0;i<n;i++) {
		for (j=s[i]+1;j<='z';j++) 
			l+=f[j];
		f[s[i]]++;
	}
	
	fout<<l<<"\n";		
	
	
	return 0;
}