Equal Elements

An array $B$ of size $2 \cdot M$ is called good if the following condition holds: - For each $i = 1, 2, \ldots, M$, $B_{2 \cdot i - 1} = B_{2 \cdot i}$. For example, $B = [1, 1, 3, 3]$ and $B = [4, 4]$ are good, but $B = [1, 1, 2]$ is not. --- You are given an array $A$ of length $N$. Find the length of longest subsequence of $A$ which is good. A subsequence is formed by deleting several ele