CF578A A Problem about Polyline

There is a polyline going through points $ (0,0)–(x,x)–(2x,0)–(3x,x)–(4x,0)–...-(2kx,0)–(2kx+x,x)–... $ . We know that the polyline passes through the point $ (a,b) $ . Find minimum positive value $ x $ such that it is true or determine that there is no such $ x $ .

Only one line containing two positive integers $ a $ and $ b $ ( $ 1

Output the only line containing the answer. Your answer will be considered correct if its relative or absolute error doesn't exceed $ 10^{-9} $ . If there is no such $ x $ then output $ -1 $ as the answer.

You can see following graphs for sample 1 and sample 3.