E - Cascading Operations

You are given a string $S$ of length $N$ consisting of lower case English characters only. It is guaranteed that $S_i \ne S_{i + 1}$ for all $1 \le i < N$. You must insert a single lowercase character somewhere into the string. Afterwards, the following process happens: - While there exists an index $i$ ($1 \le i < |S|$) such that $S_i = S_{i + 1}$, such an index $i$ is found and we delete both