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

fn check(p: Vec<(usize, usize)>) -> bool {
    let len = p.len();
    let mut current = p[0].0;
    for i in 1..len {
        if current <= p[i].1  {
            current = current.max(p[i].0);
        } else {
            return false;
        }
    }
    return true;
}
fn main() {
  input!{
    n: usize,
    mut lr: [(usize, usize); n]
  }
  let mut ans = 0;
  for prm in lr.into_iter().permutations(n) {
    if check(prm) {ans+=1;}
  }
  println!("{:?}", ans);
}