The numbers $(a_1 , a_2 , \ldots, a_{2021})$ and $(b_1 , b_2 , \ldots, b_{2021})$ are some different permutations of the numbers $(1, 2, \ldots, 2021)$, and the numbers $(c_1 , c_2 , \ldots, c_{2021})$ are some permutation of the numbers $(2, 4, \ldots, 4042)$. Prove that the given number D is positive: $$D = \frac{c_1^2 -4 a_1b_1}{a_1 + b_1 + c_1} + \frac{c_2^2 - 4a_2b_2}{a_2 + b_2 + c_2} + \ldots + \frac{c_{2021}^2 - 4a_{2021}b_{2021}}{a_{2021} + b_{2021} + c_{2021}}$$ Proposed by Bogdan Rublov