// ---------- begin ModInt ----------
const MOD: u32 = 1_000_000_007;

#[derive(Clone, Copy)]
struct ModInt(u32);

impl std::ops::Add for ModInt {
    type Output = ModInt;
    fn add(self, rhs: ModInt) -> Self::Output {
        let mut d = self.0 + rhs.0;
        if d >= MOD {
            d -= MOD;
        }
        ModInt(d)
    }
}

impl std::ops::AddAssign for ModInt {
    fn add_assign(&mut self, rhs: ModInt) {
        *self = *self + rhs;
    }
}

impl std::ops::Sub for ModInt {
    type Output = ModInt;
    fn sub(self, rhs: ModInt) -> Self::Output {
        let mut d = self.0 + MOD - rhs.0;
        if d >= MOD {
            d -= MOD;
        }
        ModInt(d)
    }
}

impl std::ops::SubAssign for ModInt {
    fn sub_assign(&mut self, rhs: ModInt) {
        *self = *self - rhs;
    }
}

impl std::ops::Mul for ModInt {
    type Output = ModInt;
    fn mul(self, rhs: ModInt) -> Self::Output {
        ModInt((self.0 as u64 * rhs.0 as u64 % MOD as u64) as u32)
    }
}

impl std::ops::MulAssign for ModInt {
    fn mul_assign(&mut self, rhs: ModInt) {
        *self = *self * rhs;
    }
}

impl std::ops::Neg for ModInt {
    type Output = ModInt;
    fn neg(self) -> Self::Output {
        ModInt(if self.0 == 0 {0} else {MOD - self.0})
    }
}

impl std::fmt::Display for ModInt {
    fn fmt<'a>(&self, f: &mut std::fmt::Formatter<'a>) -> std::fmt::Result {
        write!(f, "{}", self.0)
    }
}

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

impl From<usize> for ModInt {
    fn from(val: usize) -> Self {
        ModInt((val % MOD as usize) as u32)
    }
}

#[allow(dead_code)]
impl ModInt {
    pub fn new(n: u32) -> ModInt {
        ModInt(n % MOD)
    }
    pub fn zero() -> ModInt {
        ModInt(0)
    }
    pub fn one() -> ModInt {
        ModInt(1)
    }
    pub fn pow(self, mut n: u32) -> ModInt {
        let mut t = ModInt::one();
        let mut s = self;
        while n > 0 {
            if n & 1 == 1 {
                t *= s;
            }
            s *= s;
            n >>= 1;
        }
        t
    }
    pub fn inv(self) -> ModInt {
        assert!(self.0 > 0);
        self.pow(MOD - 2)
    }
}
// ---------- end ModInt ----------
// ---------- begin Precalc ----------
#[allow(dead_code)]
struct Precalc {
    inv: Vec<ModInt>,
    fact: Vec<ModInt>,
    ifact: Vec<ModInt>,
}

#[allow(dead_code)]
impl Precalc {
    pub fn new(n: usize) -> Precalc {
        let mut inv = vec![ModInt::one(); n + 1];
        let mut fact = vec![ModInt::one(); n + 1];
        let mut ifact = vec![ModInt::one(); n + 1];
        for i in 2..(n + 1) {
            inv[i] = -inv[MOD as usize % i] * ModInt(MOD / i as u32);
            fact[i] = fact[i - 1] * ModInt(i as u32);
            ifact[i] = ifact[i - 1] * inv[i];
        }
        Precalc {
            inv: inv,
            fact: fact,
            ifact: ifact,
        }
    }
    pub fn inv(&self, n: usize) -> ModInt {
        self.inv[n]
    }
    pub fn fact(&self, n: usize) -> ModInt {
        self.fact[n]
    }
    pub fn ifact(&self, n: usize) -> ModInt {
        self.ifact[n]
    }
    pub fn comb(&self, n: usize, k: usize) -> ModInt {
        if k > n {
            return ModInt::zero();
        }
        self.fact[n] * self.ifact[k] * self.ifact[n - k]
    }
}
// ---------- end Precalc ----------

use std::io::Read;

// 解説みた
// いい感じにDPできるとのことだがどういうことだろう
// T が1種類だったら掛け算で計算可能なのでこれを使いそう
// Xについてソートしておく
// T = 0 の条件をすべて満たしつつT = 1 の条件をk個満たさないものの数を数える
// これはすべて <= X という形の条件のみなので掛け算で計算可能
// dp[i][k] : Xが小さい方からi個見て、T=0の条件はすべて満たしつつT=1の条件をk個満たさないものの数とおく
// cnt_i : i番目までの条件でT=0のものの数
// とおくと
// T_i = 0 のとき
// dp[i][k] = dp[i - 1][k] * (X_i - cnt - k)
// T_i = 1 のとき
// dp[i][k] = dp[i - 1][k - 1] * (X_i - (cnt + k - 1)) + dp[i - 1][k]
// とかける
// 最後に階乗を掛け合わせて包除
fn run() {
    let mut s = String::new();
    std::io::stdin().read_to_string(&mut s).unwrap();
    let mut it = s.trim().split_whitespace();
    let n: usize = it.next().unwrap().parse().unwrap();
    let mut cond = vec![];
    for _ in 0..n {
        let t: usize = it.next().unwrap().parse().unwrap();
        let x: usize = it.next().unwrap().parse().unwrap();
        cond.push((x - t, t));
    }
    cond.sort();
    let mut dp = vec![ModInt::zero(); n + 1];
    dp[0] = ModInt::one();
    let mut cnt = 0;
    for &(x, t) in cond.iter() {
        if t == 0 {
            let mut next = vec![ModInt::zero(); n + 1];
            for (j, v) in dp.into_iter().enumerate() {
                if j + cnt >= x {
                    break;
                }
                next[j] = v * ModInt::from(x - j - cnt);
            }
            dp = next;
            cnt += 1;
        } else {
            for j in (0..n).rev() {
                if x <= j + cnt {
                    continue;
                }
                let v = dp[j];
                dp[j + 1] += v * ModInt::from(x - j - cnt);
            }
        }
    }
    let mut ans = ModInt::zero();
    let pc = Precalc::new(n);
    let mut sign = ModInt::one();
    for (i, v) in dp.into_iter().enumerate().take(n + 1 - cnt) {
        ans += sign * pc.fact(n - cnt - i) * v;
        sign = -sign;
    }
    println!("{}", ans);
}

fn main() {
    run();
}