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

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

    let mut ans = 0;
    'outer: for lr in lr.iter().permutations(n) {
        let mut diff = 0;

        for &(l, r) in lr {
            if diff > r {
                continue 'outer;
            }

            diff = diff.max(l);
        }

        ans += 1;
    }
    println!("{}", ans);
}