use proconio::{fastout, input};

#[fastout]
fn main() {
    input! {
        n: usize,
		diff: [(u16, u16); n],
    }

    println!("{}", output(solve(n, diff)));
}

fn solve(n: usize, diff: Vec<(u16, u16)>) -> u16 {
	struct Fact<'a> {
		f: &'a dyn Fn(&Fact, u16, u8) -> u16,
	}

	let dfs = Fact {
		f: &|fact, prev_diff, flag| -> u16 {
			if flag as u16 == (1 << n) - 1 {
				return 1;
			}
		
			let mut ans = 0;
			for i in 0..n {
				let (l, r) = diff[i];
				if (flag >> i) & 1 == 0 && prev_diff <= r {
					ans += (fact.f)(&fact, prev_diff.max(l), flag | (1 << i));
				}
			}
			ans
		},
	};
    (dfs.f)(&dfs, 0, 0)
}

fn output(ans: u16) -> u16 {
    ans
}