Guide Ted

Ted needs to cross an $X*Y$ grid. He is currently at position $(0,0)$. He needs to reach the position $(x,y)$. The only possible moves allowed are right $[(a,b) to (a+1,b)]$ and up $[(a,b) to (a,b+1)]$. However, there a pothole in the grid at any random location $(A,B)$. Ted must not step on this position lest he should fall in it and break some bones. You need to tell Ted the number of possible