var a:array[1..10000] of char;
    fr:array['a'..'z'] of integer;
    x:char;
    s,sc,i,n:longint;
    f,g:text;
begin
     assign(f,'litere.in'); reset(f);
     assign(g,'litere.out'); rewrite(g);
     readln(f,n);
     for i:=1 to n do read(f,a[i]);
     inc(fr[a[1]]);       s:=0;
     for i:=2 to n do
     begin
          sc:=0;
          for x:=chr(ord(a[i])+1) to 'z' do sc:=sc+fr[x];
          inc(fr[a[i]]);
          s:=s+sc;
     end;
     writeln(g,s);
     close(g);
end.