|Codeforces Round #179 (Div. 1)|
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:
Greg wondered, how many ways there are to paint a cave on his pad. Two ways can be considered distinct if there is a cell that has distinct colors on the two pictures.
The first line contains two integers n, m — the pad's screen size (1 ≤ n, m ≤ 2000).
In the single line print the remainder after dividing the answer to the problem by 1000000007 (109 + 7).