var f,g:text;
    n,i,j,k:integer;
    aux:char;
    a:array[1..1000]of char;
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]);
     k:=0;
     for i:=1 to n-1 do
         for j:=i+1 to n do
             if a[i]>a[j] then begin
                               k:=k+1;
                               aux:=a[i];
                               a[i]:=a[j];
                               a[j]:=aux;
                               end;
     writeln(g,k);
     close(f); close(g);
end.