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 Rust (1.15.1) (judge verdict: Accepted)

#![allow(unused_imports)]
#![allow(non_snake_case)]
use std::cmp::*;
use std::collections::*;
use std::io::Write;

#[allow(unused_macros)]
macro_rules! debug {
    ($($e:expr),*) => {
        #[cfg(debug_assertions)]
        $({
            let (e, mut err) = (stringify!($e), std::io::stderr());
            writeln!(err, "{} = {:?}", e, $e).unwrap()
        })*
    };
}

fn main() {
    let v = read_vec::<i64>();
    let (h, w, a, b) = (v[0], v[1], v[2], v[3]);
    let big_int = 1000_000_000 + 7;

    let fact = ModFactorial::new(200000, big_int).fact;
    let inv_fact = (0..fact.len())
        .map(|x| mod_inverse(fact[x as usize], big_int))
        .collect::<Vec<_>>();

    let mut ans = 0;
    let mut temp1 = 1;
    let res_h = h - 1;
    let res_w = w - b - 1;
    let mut temp2 = ModBinominal::new(res_h + res_w, big_int, &fact, &inv_fact).bin[res_h as usize];
    for i in 0..h - a {
        ans += (temp1 * temp2) % big_int;
        ans %= big_int;
        temp1 *= b + i;
        temp1 %= big_int;
        temp1 *= mod_inverse(i + 1, big_int);
        temp1 %= big_int;

        temp2 *= res_h - i;
        temp2 %= big_int;
        temp2 *= mod_inverse(res_h + res_w - i, big_int);
        temp2 %= big_int;
    }
    println!("{:?}", ans);
}

fn read<T: std::str::FromStr>() -> T {
    let mut s = String::new();
    std::io::stdin().read_line(&mut s).ok();
    s.trim().parse().ok().unwrap()
}

fn read_vec<T: std::str::FromStr>() -> Vec<T> {
    read::<String>()
        .split_whitespace()
        .map(|e| e.parse().ok().unwrap())
        .collect()
}

fn extgcd(a: i64, b: i64, x: &mut i64, y: &mut i64) -> i64 {
    let mut d = a;
    if b == 0 {
        *x = 1;
        *y = 0;
    } else {
        d = extgcd(b, a % b, y, x);
        *y -= (a / b) * (*x);
    }
    return d;
}

fn mod_inverse(a: i64, m: i64) -> i64 {
    let mut x: i64 = 0;
    let mut y: i64 = 0;
    extgcd(a, m, &mut x, &mut y);
    (m + x % m) % m
}

struct ModFactorial {
    fact: Vec<i64>,
}

// returns n! under mod m
impl ModFactorial {
    fn new(n: i64, m: i64) -> ModFactorial {
        let mut fact = vec![1; n as usize + 1];
        for i in 2..fact.len() {
            fact[i] = (i as i64 * fact[i - 1]) % m;
        }
        ModFactorial { fact: fact }
    }
}

// returns bin[i] = nCi under mod m
struct ModBinominal {
    bin: Vec<i64>,
}

impl ModBinominal {
    fn new(n: i64, m: i64, fact: &Vec<i64>, inv_fact: &Vec<i64>) -> ModBinominal {
        let mut bin = vec![1; n as usize + 1];
        if n == 0 {
            return ModBinominal { bin: bin };
        }
        for i in 1..n + 1 {
            let a2 = inv_fact[i as usize] % m;
            let a3 = inv_fact[(n - i) as usize] % m;
            bin[i as usize] = fact[n as usize] * ((a2 * a3) % m) % m;
        }
        ModBinominal { bin: bin }
    }
}