CF627A XOR Equation

Two positive integers $ a $ and $ b $ have a sum of $ s $ and a bitwise XOR of $ x $ . How many possible values are there for the ordered pair $ (a,b) $ ?

The first line of the input contains two integers $ s $ and $ x $ ( $ 2

Print a single integer, the number of solutions to the given conditions. If no solutions exist, print $ 0 $ .

In the first sample, we have the following solutions: $ (2,7) $ , $ (3,6) $ , $ (6,3) $ , $ (7,2) $ . In the second sample, the only solutions are $ (1,2) $ and $ (2,1) $ .