Iroha and a Grid(AtCoder

Iroha and a Grid(AtCoder"/>

Iroha and a Grid(AtCoder

题目描述
We have a large square grid with H rows and W columns. Iroha is now standing in the top-left cell. She will repeat going right or down to the adjacent cell, until she reaches the bottom-right cell.
However, she cannot enter the cells in the intersection of the bottom A rows and the leftmost B columns. (That is, there are A×B forbidden cells.) There is no restriction on entering the other cells.
Find the number of ways she can travel to the bottom-right cell.
Since this number can be extremely large, print the number modulo 109+7.

Constraints
1≤H,W≤100,000
1≤A<H
1≤B<W

输入
The input is given from Standard Input in the following format:

H W A B

输出
Print the number of ways she can travel to the bottom-right cell, modulo 109+7.

样例输入
2 3 1 1

样例输出
2

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