Determine for which positive integers the set can be partitioned into two disjoint subsets and such that the sum of the elements of is equal to the sum of the elements of
%V0
Determine for which positive integers $k$ the set $$X = \{1990, 1990 + 1, 1990 + 2, \ldots, 1990 + k\}$$ can be partitioned into two disjoint subsets $A$ and $B$ such that the sum of the elements of $A$ is equal to the sum of the elements of $B.$
Izvor: Međunarodna matematička olimpijada, shortlist 1990
Školjka 
the set 
can be partitioned into two disjoint subsets 
and 
such that the sum of the elements of 