Let $$f(x)=\left(\dfrac{1}{3}x^3-x^2+\dfrac{2}{3}\right)\cos^{2017}{\left(\dfrac{\pi}{3}x+\dfrac{2\pi}{3}\right)}+2x+3,~~-2015\le x\le 2017$$
Assmue that the maximum if $M$,and the minimum of the value is $m$.Find the $M+m=( ~~)$
$A:5~~~~~~~~~~~~~~~$ $B: 10~~~~~~~~~~~~~~~~~~~$$C:~1~~~~~~~~~~~~~~~~~~~$$D:0~~~~~~$
Maybe this problem have some nice relation?because we can't to find the maxium of the value and also the minimum value