#include <fstream.h>
char sir[10002];
long long n,S,a[26];
void citire()
{
   ifstream fin("litere.in");
   fin>>n;
   for (int i=0;i<n;i++ )
      fin>>sir[i];
   fin.close();
}

void numarare()
{
   for (int i=0;i<n;i++){
      for (int j=sir[i]-'a'+1;j<=26;j++)
	 if (a[j]!=0)
	    S+=a[j];
    a[sir[i]-'a']++;
    }
}

int main()
{
   citire();
   numarare();
   ofstream fout("litere.out");
   fout<<S;
   fout<<"\n";
   fout.close();
   return 0;
}