Program litere;
Var f,ff:text;
    n,i,l,aux:longint;
    h:array['a'..'z'] of longint;
    a,aa,j:char;
Begin
 assign(f,'litere.in');
 reset(f);
 assign(ff,'litere.out');
 rewrite(ff);
 readln(f,n);
 l:=0;
 for i:=1 to n do
    begin
     read(f,a);
     inc(h[a]);
     for j:=chr(ord(a)+1) to 'z' do
        inc(l,h[j]);
    end;
 {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.