#include<iostream.h>
#include<string.h>
#include<fstream.h>
fstream f("litere.in",ios::in);
fstream g("litere.out",ios::out);

int N,k,i;
char ch,c;
int Ap[133];
int main()
{
  for (i=0;i<=133;i++)
	Ap[i]=0;

  f>>N;
  k=0;
  for(i=1;i<=N;i++)
  {
     f>>ch;
     Ap[ch]++;
     for (c=ch+1;c<='z';c++)
       if (Ap[c]) k=k+Ap[c];
  }
  f.close();
  g<<k;
  g.close();
  return 0;
}