#include<fstream>

using namespace std;

ifstream in("litere.in");
ofstream out("litere.out");

char s[10004];

int main()
{
   int i,j,n,c=0,contor=0;
   in>>n>>ws;
   in.getline(s,10004);   
   for (i=1;i<27;i++)
   {
	   c=0;
	   for(j=0;j<n;j++)
		   if(s[j]=='a'+i-1) 
			   contor += c;
		   else 
			   if(s[j]>'a'+i-1) 
				   c++;
   }
   out<<contor<<'\n';
   return 0;
}