Flying Cabs

Now there is a trend of flying cabs comming. But the technology is under development and the cabs can only move forward or backward a fix number of steps $x$. There are $N$ stations(where the cab needs to stops to board and unboard passengers). All the stations are in a straight line. For each valid $i$, The $i$-th station is located at $A$$i$ position. If Cab is at a location $R$, then it can m