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)

#pragma warning disable

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Runtime.Serialization.Formatters.Binary;
using System.Text;
using System.Text.RegularExpressions;
using System.Diagnostics;
using System.Numerics;
using System.Collections;

static class MainClass
{
    public static void Main(string[] args)
    {
        var hwab = Console.ReadLine().SplittedStringToInt32List();

        var h = hwab[0];
        var w = hwab[1];
        var a = hwab[2];
        var b = hwab[3];
        SetFactor(h + w);
        var nums = h - a;

        BigInteger counts = 0;
        for (int i = 1; i <= h - a; i++)
        {
            counts %= 1000000007;
            counts += (GetCombination(i + b - 2, i - 1) % 1000000007) * (GetCombination(w - b + h - 1 - i, w - b - 1) % 1000000007);
        }
        Console.WriteLine(counts % Mod1e9);

        Console.ReadLine();
    }

    // 階乗
    public static BigInteger Factorial(int n)
    {
        var dp = new BigInteger[n];
        dp[0] = 1;
        for (int i = 1; i < n; i++)
        {
            dp[i] = dp[i - 1] * (i + 1);
        }
        return dp[n - 1];
    }
    public static BigInteger[] factors;
    public static BigInteger[] revFactors;
    public static void SetFactor(int N)
    {
        factors = new BigInteger[N];
        factors[0] = 1;
        revFactors = new BigInteger[N];
        for (int i = 1; i < N; i++)
        {
            factors[i] = factors[i - 1] * i;
            factors[i] %= Mod1e9;
        }
        revFactors[N - 1] = BigInteger.ModPow(factors[N - 1], Mod1e9 - 2, Mod1e9);
        for (int i = N - 2; i >= 0; i--)
        {
            revFactors[i] = revFactors[i + 1] * (i + 1);
            revFactors[i] %= Mod1e9;
        }
    }
    public static BigInteger GetCombination(int a,int b)
    {
        return (((factors[a] * revFactors[b]) % Mod1e9) * revFactors[a - b]) % Mod1e9;
    }

    //public static BigInteger Combination(int n, int k)
    //{
    //    if (n == k)
    //        return 1;
    //    if (n < k || n == 0)
    //        return 0;
        
    //    try
    //    {
    //        return Factorial(n) / (Factorial(n - k) * Factorial(k));
    //    }
    //    catch
    //    {
    //        return 0;
    //    }
    //}


    #region ライブラリ
    public static long ToInt64(this string str) => long.Parse(str);
    public static int ToInt32(this string str) => int.Parse(str);
    public static BigInteger ToBigInteger(this string str) => BigInteger.Parse(str);
    public static List<string> SplittedStringToList(this string str) => str.Split(new[] { ' ' }, StringSplitOptions.RemoveEmptyEntries).ToList();
    public static List<int> SplittedStringToInt32List(this string str) => str.Split(new[] { ' ' }, StringSplitOptions.RemoveEmptyEntries).Select(x => int.Parse(x)).ToList();
    public static List<long> SplittedStringToInt64List(this string str) => str.Split(new[] { ' ' }, StringSplitOptions.RemoveEmptyEntries).Select(x => long.Parse(x)).ToList();
    public static List<BigInteger> SplittedStringToBigInteger(this string str) => str.Split(new[] { ' ' }, StringSplitOptions.RemoveEmptyEntries).Select(x => BigInteger.Parse(x)).ToList();
    public const int INF = int.MaxValue / 2;
    public const long LONGINF = long.MaxValue / 2;
    public const int Mod1e9 = 1000000007;

        // 素因数分解
    public static List<long> PrimeFactorize(long n)
    {
        var bases = n;
        var num = (long)Math.Pow(n, 0.5);
        var ls = new List<long>();
        for (int i = 2; i <= num; i++)
        {
            if (n % i == 0)
            {
                n = n / i;
                ls.Add(i);
                i--;
                continue;
            }
        }
        long nnn = 1;
        foreach (var item in ls)
        {
            nnn = nnn * item;
        }
        if (nnn != bases)
            ls.Add(n);
        return ls;
    }


    public static List<long> MakeDivisors(long n)
    {
        var ls = new List<long>();
        // ls.Add(1);
        for (int i = 1; i * i <= n; i++)
        {
            if (n % i == 0)
            {
                ls.Add(i);
                if (i != 1 && i * i != n)
                {
                    ls.Add(n / i);
                }
            }
        }
        return ls;
    }

    public static void PrintArray(int[,] array)
    {
        for (int i = 0; i < array.GetLength(0); i++)
        {
            var sb = new StringBuilder();
            for (int j = 0; j < array.GetLength(1); j++)
            {
                sb.Append(array[i, j]).Append(" ");
            }
            Console.WriteLine(sb.ToString());
        }
    }

    #endregion
}