Safe Robots

There are two robots, one belonging to Alice, and the other to Bob. These robots are currently standing in a lane. You are given this information by a string $s$, whose characters are either '.' denoting empty cell, 'A' denoting Alice's robot, or 'B' denoting Bob's robot. We guarantee that Alice's robot lies to the left of Bob's robot. You are given two integers $sa$, and $sb$. Alice's robot move