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.Collections.Generic;
using System.Linq;
using static System.Console;
using static System.Convert;
using static System.Math;
//using static System.Globalization.CultureInfo;

class Program
{
    static void Main(string[] args)
    {
        var hwab = Input.ar;
        var com = new Calculation.Combination(hwab[0] + hwab[1]);
        var sum = 0L;
        for(var i = hwab[3]; i < hwab[1]; i++)
        {
            sum += (long)com.Comb(hwab[0] - hwab[2] + i - 1, i) * com.Comb(hwab[2] + hwab[1] - i - 2, hwab[1] - i - 1);
            sum %= Input.MOD;
        }
        WriteLine(sum);
    }
}

public class Input
{
    public static string read => ReadLine();
    public  static int[] ar => Array.ConvertAll(read.Split(' '), int.Parse);
    public  static int num => ToInt32(read);
    public static long[] arL => Array.ConvertAll(read.Split(' '), long.Parse);
    public  static long numL => ToInt64(read);
    public static int[][] twodim(int num)
        => Enumerable.Repeat(0, num).Select(v => ar).ToArray();
    public static long[][] twodimL(int num)
        => Enumerable.Repeat(0, num).Select(v => arL).ToArray();
    public  const string Alfa = "abcdefghijklmnopqrstuvwxyz";
    public  const int MOD = 1000000007;
}

public class Calculation
{
    public static int LCM(int num1, int num2)
    {
        return num1 / GCD(num1, num2) * num2;
    }

    public static long LCM(long num1, long num2)
    {
        return num1 / GCD(num1, num2) * num2;
    }

    public static int GCD(int num1, int num2)
    {
        return num1 < num2 ? GCD(num2, num1) :
            num2 > 0 ? GCD(num2, num1 % num2) : num1;
    }

    public static long GCD(long num1, long num2)
    {
        return num1 < num2 ? GCD(num2, num1) :
            num2 > 0 ? GCD(num2, num1 % num2) : num1;
    }

    public static int Multiple(int num1, int num2)
        => (int)(BigMul(num1, num2) % Input.MOD);
    
    public static int Pow(int m,int n)
    {
        if (n == 0) return 1;
        if (n % 2 == 0) return Pow(Multiple(m, m), n / 2);
        else return Multiple(Pow(Multiple(m, m), n / 2), m);
    }

    public static int Div(int a, int b)
        => Multiple(a, Pow(b, Input.MOD - 2));

    public class Combination
    {
        private int[] _fac;
        public Combination(int num)//階乗をメモ化
        {
            _fac = new int[num + 1];
            _fac[0] = 1;
            for (var i = 1; i <= num; i++)
                _fac[i] = Multiple(_fac[i - 1], i);
        }
        public int Comb(int n,int r)
        {
            if (n < r) return 0;
            if (n == r) return 1;
            var calc = _fac[n];
            calc = Div(calc, _fac[r]);
            calc = Div(calc, _fac[n - r]);
            return calc;
        }
    }
}