Problem:
Score : 100 points

Problem Statement

There is a grid with H horizontal rows and W vertical columns.
Let (i, j) denote the square at the i-th row from the top and the j-th column from the left.

In the grid, N Squares (r_1, c_1), (r_2, c_2), \ldots, (r_N, c_N) are wall squares, and the others are all empty squares.
It is guaranteed that Squares (1, 1) and (H, W) are empty squares.

Taro will start from Square (1, 1) and reach (H, W) by repeatedly moving right or down to an adjacent empty square.

Find the number of Taro's paths from Square (1, 1) to (H, W), modulo 10^9 + 7.

Constraints

All values in input are integers.

2 \leq H, W \leq 10^5

1 \leq N \leq 3000

1 \leq r_i \leq H

1 \leq c_i \leq W

Squares (r_i, c_i) are all distinct.

Squares (1, 1) and (H, W) are empty squares.

Input

Input is given from Standard Input in the following format:

H W N
r_1 c_1
r_2 c_2
:
r_N c_N

Output

Print the number of Taro's paths from Square (1, 1) to (H, W), modulo 10^9 + 7.

Sample Input 1

3 4 2
2 2
1 4

Sample Output 1

3

There are three paths as follows:

Sample Input 2

5 2 2
2 1
4 2

Sample Output 2

0

There may be no paths.

Sample Input 3

5 5 4
3 1
3 5
1 3
5 3

Sample Output 3

24

Sample Input 4

100000 100000 1
50000 50000

Sample Output 4

123445622

Be sure to print the count modulo 10^9 + 7.

Solution in Rust (1.42.0) (judge verdict: Accepted)

mod modint {

    #[allow(dead_code)]
    pub struct Mod;
    impl ConstantModulo for Mod {
        const MOD: u32 = 1_000_000_007;
    }

    #[allow(dead_code)]
    pub struct RuntimeMod;
    static mut RUNTIME_MOD: u32 = 0;
    impl Modulo for RuntimeMod {
        fn modulo() -> u32 {
            unsafe { RUNTIME_MOD }
        }
    }

    #[allow(dead_code)]
    impl RuntimeMod {
        pub fn set_modulo(p: u32) {
            unsafe {
                RUNTIME_MOD = p;
            }
        }
    }

    use std::marker::*;
    use std::ops::*;

    pub trait Modulo {
        fn modulo() -> u32;
    }

    pub trait ConstantModulo {
        const MOD: u32;
    }

    impl<T> Modulo for T
    where
        T: ConstantModulo,
    {
        fn modulo() -> u32 {
            T::MOD
        }
    }

    pub struct ModularInteger<T>(pub u32, PhantomData<T>);

    impl<T> Clone for ModularInteger<T> {
        fn clone(&self) -> Self {
            ModularInteger::new_unchecked(self.0)
        }
    }

    impl<T> Copy for ModularInteger<T> {}

    impl<T: Modulo> Add for ModularInteger<T> {
        type Output = ModularInteger<T>;
        fn add(self, rhs: Self) -> Self::Output {
            let mut d = self.0 + rhs.0;
            if d >= T::modulo() {
                d -= T::modulo();
            }
            ModularInteger::new_unchecked(d)
        }
    }

    impl<T: Modulo> AddAssign for ModularInteger<T> {
        fn add_assign(&mut self, rhs: Self) {
            *self = *self + rhs;
        }
    }

    impl<T: Modulo> Sub for ModularInteger<T> {
        type Output = ModularInteger<T>;
        fn sub(self, rhs: Self) -> Self::Output {
            let mut d = T::modulo() + self.0 - rhs.0;
            if d >= T::modulo() {
                d -= T::modulo();
            }
            ModularInteger::new_unchecked(d)
        }
    }

    impl<T: Modulo> SubAssign for ModularInteger<T> {
        fn sub_assign(&mut self, rhs: Self) {
            *self = *self - rhs;
        }
    }

    impl<T: Modulo> Mul for ModularInteger<T> {
        type Output = ModularInteger<T>;
        fn mul(self, rhs: Self) -> Self::Output {
            let v = self.0 as u64 * rhs.0 as u64 % T::modulo() as u64;
            ModularInteger::new_unchecked(v as u32)
        }
    }

    impl<T: Modulo> MulAssign for ModularInteger<T> {
        fn mul_assign(&mut self, rhs: Self) {
            *self = *self * rhs;
        }
    }

    impl<T: Modulo> Neg for ModularInteger<T> {
        type Output = ModularInteger<T>;
        fn neg(self) -> Self::Output {
            if self.0 == 0 {
                Self::zero()
            } else {
                Self::new_unchecked(T::modulo() - self.0)
            }
        }
    }

    impl<T> std::fmt::Display for ModularInteger<T> {
        fn fmt<'a>(&self, f: &mut std::fmt::Formatter<'a>) -> std::fmt::Result {
            write!(f, "{}", self.0)
        }
    }

    impl<T: Modulo> std::str::FromStr for ModularInteger<T> {
        type Err = std::num::ParseIntError;
        fn from_str(s: &str) -> Result<Self, Self::Err> {
            let val = s.parse::<u32>()?;
            Ok(ModularInteger::new(val))
        }
    }

    impl<T: Modulo> From<usize> for ModularInteger<T> {
        fn from(val: usize) -> ModularInteger<T> {
            ModularInteger::new_unchecked((val % T::modulo() as usize) as u32)
        }
    }

    impl<T: Modulo> From<i64> for ModularInteger<T> {
        fn from(val: i64) -> ModularInteger<T> {
            let m = T::modulo() as i64;
            ModularInteger::new((val % m + m) as u32)
        }
    }

    #[allow(dead_code)]
    impl<T> ModularInteger<T> {
        fn new_unchecked(d: u32) -> Self {
            ModularInteger(d, PhantomData)
        }
        pub fn zero() -> Self {
            ModularInteger::new_unchecked(0)
        }
        pub fn one() -> Self {
            ModularInteger::new_unchecked(1)
        }
        pub fn is_zero(&self) -> bool {
            self.0 == 0
        }
    }

    #[allow(dead_code)]
    impl<T: Modulo> ModularInteger<T> {
        pub fn new(d: u32) -> Self {
            ModularInteger::new_unchecked(d % T::modulo())
        }
        pub fn pow(&self, mut n: u32) -> Self {
            let mut t = Self::one();
            let mut s = *self;
            while n > 0 {
                if n & 1 == 1 {
                    t *= s;
                }
                s *= s;
                n >>= 1;
            }
            t
        }
        pub fn inv(&self) -> Self {
            assert!(self.0 != 0);
            self.pow(T::modulo() - 2)
        }
    }

    // ---------- begin Precalc ----------
    #[allow(dead_code)]
    pub struct Precalc<T> {
        inv: Vec<ModularInteger<T>>,
        fact: Vec<ModularInteger<T>>,
        ifact: Vec<ModularInteger<T>>,
    }

    #[allow(dead_code)]
    impl<T: Modulo> Precalc<T> {
        pub fn new(n: usize) -> Precalc<T> {
            let mut inv = vec![ModularInteger::one(); n + 1];
            let mut fact = vec![ModularInteger::one(); n + 1];
            let mut ifact = vec![ModularInteger::one(); n + 1];
            for i in 2..(n + 1) {
                fact[i] = fact[i - 1] * ModularInteger::new_unchecked(i as u32);
            }
            ifact[n] = fact[n].inv();
            if n > 0 {
                inv[n] = ifact[n] * fact[n - 1];
            }
            for i in (1..n).rev() {
                ifact[i] = ifact[i + 1] * ModularInteger::new_unchecked((i + 1) as u32);
                inv[i] = ifact[i] * fact[i - 1];
            }
            Precalc {
                inv: inv,
                fact: fact,
                ifact: ifact,
            }
        }
        pub fn inv(&self, n: usize) -> ModularInteger<T> {
            assert!(n > 0);
            self.inv[n]
        }
        pub fn fact(&self, n: usize) -> ModularInteger<T> {
            self.fact[n]
        }
        pub fn ifact(&self, n: usize) -> ModularInteger<T> {
            self.ifact[n]
        }
        pub fn perm(&self, n: usize, k: usize) -> ModularInteger<T> {
            if k > n {
                ModularInteger::zero()
            } else {
                self.fact[n] * self.ifact[n - k]
            }
        }
        pub fn comb(&self, n: usize, k: usize) -> ModularInteger<T> {
            if k > n {
                return ModularInteger::zero();
            }
            self.fact[n] * self.ifact[k] * self.ifact[n - k]
        }
    }
    // ---------- end Precalc ----------
}

use modint::*;
use proconio::marker::*;
use proconio::*;

type ModInt = ModularInteger<Mod>;

#[fastout]
fn run() {
    input! {
        h: usize,
        w: usize,
        n: usize,
        mut p: [(Usize1, Usize1); n],
    }
    p.push((h - 1, w - 1));
    p.sort();
    let mut dp = vec![ModInt::zero(); p.len()];
    let pc = Precalc::new(h + w);
    for (i, &(x, y)) in p.iter().enumerate() {
        let mut way = pc.comb(x + y, x);
        p.iter()
            .zip(dp.iter())
            .take(i)
            .filter(|(p, _)| p.0 <= x && p.1 <= y)
            .for_each(|(&(a, b), &v)| {
                way -= v * pc.comb(x + y - a - b, x - a);
            });
        dp[i] = way;
    }
    let ans = dp[n];
    println!("{}", ans);
}

fn main() {
    run();
}