Insider Subsequences

You are given an array $A$ with $N$ positive integers. An $x$-insider of this array is a [subsequence](https://en.wikipedia.org/wiki/Subsequence) of $A$ such that $x$ is **strictly** in between any two consecutive elements of the subsequence. More formally, a subsequence $S = [s_0, s_1, \dots s_{k-1}]$ of the array $A$ is an $x$-insider if for every integer $i$ such that $1 \leq i \leq k-1$, ei