Neha and Destination

$Neha$ want to reach a destination but she decide that she can make moves in two directions only. If $Neha$ is at position $(x,y)$ , then she can move to $(x+1,y+1)$ or $(x+1,y)$. $Neha$ must start her journey from $(0,0)$ and her destination is $(X,Y)$ . Your task is to find the minimum number of moves that $Neha$ require to reach the destination or if she cannot reach the destination. ###Inp