package main

import . "fmt"
func main() {
	var n int
	Scan(&n)
	ls := make([]int, n)
	rs := make([]int, n)
	for i := range ls {
		Scan(&ls[i], &rs[i])
	}
	ans := 0
	for i := 0; i < (1 << (3*(n-1))); i++ {
		v := i
		s := n*(n-1)/2
		f := 0
		p := []int{}
		for j := 0; j < n-1; j++ {
			a := v&7
			v >>= 3
			p = append(p, a)
			f |= 1 << a
			s -= a
		}
		if s < 0 {
			continue
		}
		p = append(p, s)
		f |= 1 << s
		if f != (1 << n)-1 {
			continue
		}
		ok := true
		for j := range p[1:] {
			if ls[p[j]] > rs[p[j+1]] {
				ok = false
			}
		}
		if !ok {
			continue
		}
		e := ls[p[0]]
		for _, t := range p[1:] {
			if e > rs[t] {
				ok = false
			} else {
				e = max(e, ls[t])
			}
		}
		if ok {
			ans++
		}
	}
	Println(ans)
}