Magical Planks

Ryan is a boy from a small town, who has been given a task by his father. He has $N$ wooden planks, numbered from $1$ to $N$, which are colored either black or white. His task is to color all planks the same color! But there is some magic in the winds of his small town. Whenever he colors the $i^{th}$ ( plank which has the color $S_i$ ) to a color $P$ then following events happen: - if $2 \