use proconio::{fastout, input};

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

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

fn dfs(n: usize, diff: &Vec<(u16, u16)>, prev_diff: u16, flag: u8) -> 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 += dfs(n, diff, prev_diff.max(l), flag | (1 << i));
		}
	}
	ans
}

fn solve(n: usize, diff: Vec<(u16, u16)>) -> u16 {
    dfs(n, &diff, 0, 0)
}

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