use itertools::Itertools;
use proconio::input;

fn main() {
    input! {
        n: usize,
        lr: [(i64, i64); n],
    }

    println!(
        "{}",
        (0..n)
            .permutations(n)
            .fold(0, |acc, p| acc + if is_valid(&p, &lr) { 1 } else { 0 })
    );
}

fn is_valid(p: &[usize], lr: &[(i64, i64)]) -> bool {
    let mut diff = 0;
    for i in 0..p.len() {
        if diff > lr[p[i]].1 {
            return false;
        }
        diff = diff.max(lr[p[i]].0);
    }
    true
}