a = 1 + 2 + 3+ ... +2016 + 1*2*3*...*2015 = (2016·2017)/2 +
+1·2·3· ... · 1008· ... ·2015 = 1008·2017 +
+2016·3·4·5· ... ·1007·1009·1010· ... ·2015 = 1008·2017 + M₂₀₁₆ =
= 1008·(2016 + 1) +M₂₀₁₆ = 1008·2016 +1008 + M₂₀₁₆ =
= M₂₀₁₆ +1008 + M₂₀₁₆ = M₂₀₁₆ +1008