I've sold it with the long solution, but I found a shorter one and I can't understand why it is not accepted.

var
  n,tmp:longint;
begin
  readln(n);
  if n>0 then
    write((((n+1)*n)/2))
    else
      write(-(((-n+1)*-n)/2)+1);
end.

You must write:
write((((n+1)*n)/2):0:0)
or use div 2 or shr 1 instead /2