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 C# (Mono 4.6.2.0) (judge verdict: Accepted)

using System;
using System.Text;
using System.Collections.Generic;
class Solve{
    int H,W;
    int A,B;
    public Solve(){}
    StringBuilder sb;
    public static int Main(){
        new Solve().Run();
        return 0;
    }
    void Run(){
        sb = new StringBuilder();
        Read();
        Calc();
        Console.Write(sb.ToString());
    }
    void Calc(){
        Fact f = new Fact(H+W);
        long count = 0;
        for(int i=0;i<H-A;i++){
            count = (count + (f.GetConv(i+B-1,i) * f.GetConv(H-i+W-B-2,H-i-1) % Define.mod)) % Define.mod;
        }
        sb.Append(count+"\n");
    }
    void Read(){
        string[] str = Console.ReadLine().Split(' ');
        H = int.Parse(str[0]);
        W = int.Parse(str[1]);
        A = int.Parse(str[2]);
        B = int.Parse(str[3]);
    }    
}
class Fact{
    public long[] f;
    public long[] rf;
    public Fact(int N){
        f = new long[N+1];
        rf = new long[N+1];
        for(int i=0;i<N+1;i++){
            if(i == 0){
                f[i] = 1;
            }
            else{
                f[i] = (f[i-1]*i)%Define.mod;
            }
        }
        for(int i=N;i>=0;i--){
            if(i == N){
                rf[i] = pow(f[N],Define.mod-2);
            }
            else{
                rf[i] = rf[i+1]*(i+1)%Define.mod;
            }
        }
    }
    public long pow(long N,long K){
        if(K == 0){
            return 1;
        }
        else if(K % 2 == 0){
            long t = pow(N,K/2);
            return t*t%Define.mod;
        }
        else{
            return N*pow(N,K-1)%Define.mod;
        }
    }
    public long GetFact(int N){
        return f[N];
    }
    public long GetConv(int N,int R){
        return ((f[N]*rf[R])%Define.mod*rf[N-R])%Define.mod;
    }
}
public static class Define{
    public const long mod = 1000000007;
}