Program litere;
Var f,ff:text;
    n,i,l,aux:longint;
    a:array[1..11000] of char;
    aa:char;
Begin
 assign(f,'litere.in');
 reset(f);
 assign(ff,'litere.out');
 rewrite(ff);
 readln(f,n);
 for i:=1 to n do read(f,a[i]);
 l:=0;
 repeat
  aux:=0;
  for i:=1 to n-1 do
     if a[i]>a[i+1] then
       begin
        aa:=a[i];
        a[i]:=a[i+1];
        a[i+1]:=aa;
        aux:=1;
        inc(l);
       end;
 until(aux=0);
 Writeln(ff,l);
 close(f);
 close(ff);
End.