Problem:
Score : 400 points

Problem Statement

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 10^9+7.

Constraints

1 ≦ H, W ≦ 100,000

1 ≦ A < H

1 ≦ B < W

Input

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

H W A B

Output

Print the number of ways she can travel to the bottom-right cell, modulo 10^9+7.

Sample Input 1

2 3 1 1

Sample Output 1

2

We have a 2×3 grid, but entering the bottom-left cell is forbidden. The number of ways to travel is two: "Right, Right, Down" and "Right, Down, Right".

Sample Input 2

10 7 3 4

Sample Output 2

3570

There are 12 forbidden cells.

Sample Input 3

100000 100000 99999 99999

Sample Output 3

1

Sample Input 4

100000 100000 44444 55555

Sample Output 4

738162020

Solution in Python (3.4.3) (judge verdict: Accepted)

h, w, a, b = map(int, input().split())

MOD = 10**9 + 7
MAX = h+w + 5
fact = [1 for _ in range(MAX)]
finv = [1 for _ in range(MAX)]
for i in range(2, MAX):
    fact[i] = fact[i - 1] * i % MOD
    finv[i] = pow(fact[i], MOD-2, MOD)

def comb(n: int, k: int) -> int:
    if n < k or n < 0 or k < 0:
        return 0
    return (fact[n] * finv[k] % MOD) * finv[n-k] % MOD

def path(x, y):
    return comb(x+y-2, x-1)

ans = 0
for j in range(b+1, w+1):
    # print((h-a, j), (a, w-j+1))
    ans += (path(h-a, j) * path(a, w-j+1)) % MOD
    ans %= MOD
print(ans)