Longest Unfriendly Subsequence

Let's call sequence $B_1, B_2, \ldots, B_M$ **unfriendly**, if the following condition holds: - If $1 \le i < j \le M$ and $j-i \le 2$, then $B_i \neq B_j$. In other words, a sequence is **unfriendly** if any two elements on the distance at most $2$ are different. You are given a sequence $A_1, A_2, \ldots, A_N$. Find the length of its longest **unfriendly** subsequence. A sequence $C$ is a su