> program p2356;
>  var n:integer;
>      num:array[0..10000]of integer;
>      sum:array[1..10000]of integer;
> 
>  procedure init;
>   var i:integer;
>   begin
>    readln(n);
>    for i:=1 to n do
>     begin
>      readln(sum[i]);
>      num[sum[i] mod n]:=i;
>     end;
>   end;
> 
>  procedure main;
>   var i,out:integer;
>   begin
>    out:=0;
>    if num[0]<>0 then inc(out);
>    for i:=1 to n div 2 do
>     if (num[i]<>0) and (num[n-i]<>0)  and (i<>n-i)then inc(out,2);
>    writeln(out);
>    if num[0]<>0 then writeln(sum[num[0]]);
>    for i:=1 to n div 2 do
>     if (num[i]<>0) and (num[n-i]<>0) and (i<>n-i) then
>      begin
>       writeln(num[i]);
>       writeln(num[n-i]);
>      end;
>   end;
> 
>  begin
>   fillchar(num,sizeof(num),0);
>   init;
>   main;
>  end.