I've submited this code and I always get WA, then I changed the code
to simple for loops and got it accepted, but I'm still trying to
understand why this one does not work.

Anyone can help me?

var n, i, res, k: longint;
Begin
     read(input, n);
     if n<0 then k:=-n
     else k:= n;
     for i:= 1 to k do
         res:= res + i;
     if n = 0 then
        write(output, 1)
     else
        begin
        if n < 0 then
           write(output, (res - 1)* -1)
        else
           write(output, res);
        end;
End.