Maximum Plus

You are given a grid of (not necessarily non-negative) integers $G$ with $N$ rows and $M$ columns. Let's denote the integer in the cell $(i, j)$ of this grid by $G_{i,j}$; the rows and columns use 1-based indexing. Formally, let's call a non-empty subset of cells of the grid a *plus sign* if: - it is connected - there is a central cell $(i, j)$ which belongs to the subset - each other (non-centra