CF475C Kamal-ol-molk's Painting

Rumors say that one of Kamal-ol-molk's paintings has been altered. A rectangular brush has been moved right and down on the painting. Consider the painting as a $ n×m $ rectangular grid. At the beginning an $ x×y $ rectangular brush is placed somewhere in the frame, with edges parallel to the frame, ( $ 1

The first line of input contains two integers $ n $ and $ m $ , ( $ 1

Print the minimum area of the brush in a line, if the painting is possibly altered, otherwise print $ -1 $ .