Chef and Minimum Colouring

Chef placed $N$ boxes (numbered $1$ through $N$) in a straight line. For each valid $i$, the $i$-th box contains $A_i$ balls. Then, Chef painted the boxes using $M$ distinct colours in such a way that for each $M$ consecutive boxes, no two of these boxes have the same colour. Help Chef choose $M$ distinct boxes (not necessarily consecutive) such that no two of the chosen boxes have the same