CHEF AND NEIGHBOURING

Chef while playing in the ground discovered $n$ stacks of coins. The $i^{th}$ stack has ai coins. Stack $i$ and $i+1$ are neighbouring for all $1\leq i \leq n−1$. If stack i becomes empty, stack $i−1$ and $i+1$ doesn't become neighbouring. Chef is too lazy to collect all the coins, so he asked you to do the job. He allowed you to do only the following operation at a time: Select two neighbourin