Simultaneous Robots

You are given an $N\times M$ grid $A$. Let $(i,j)$ denote the square in the $i$-th row from the top and the $j$-th column from the left. Each square is either empty or blocked, denoted by characters `'.'` and `'#'`, respectively. It is guaranteed that $(1, 1)$ and $(N, M)$ are both empty. Two robots are placed initially together at $(1,1)$. The robots move under the following constraints: -