A regular polygon with 100 vertices is given. To each vertex, a natural number from the set $$\{1, 2, 3, \ldots, 49\}$$ is assigned. Prove that there are four vertices A, B, C, D such that if the numbers a, b, c, d are assigned to them respectively then $$a+b=c+d$$ and ABCD is a parallelogram.