CF295D Greg and Caves

Greg has a pad. The pad's screen is an $ n×m $ rectangle, each cell can be either black or white. We'll consider the pad rows to be numbered with integers from 1 to $ n $ from top to bottom. Similarly, the pad's columns are numbered with integers from 1 to $ m $ from left to right. Greg thinks that the pad's screen displays a cave if the following conditions hold: - There is a segment $ [l,r] $ $ (1

The first line contains two integers $ n $ , $ m $ — the pad's screen size $ (1

In the single line print the remainder after dividing the answer to the problem by $ 1000000007 $ $ (10^{9}+7) $ .