use proconio::input;

fn dfs(p: &mut Vec<(usize, usize)>, visited: &mut Vec<bool>, path: &mut Vec<usize>, num:usize, depth: usize, ans:&mut usize) {
    if depth == num{
        if check(p, path) { *ans +=1; }
        return
    }
    for u in 0..num{

        if !visited[u]{
            visited[u]=true;
            path.push(u);
            dfs(p, visited, path, num, depth + 1,ans);
            path.pop();
            visited[u]=false;
        }
    }
}

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