# frozen_string_literal: true
in_n = gets.chomp.to_i
in_lr = in_n.times.map { gets.chomp.split.map(&:to_i) }

puts (
       (0..(in_n - 1))
         .to_a
         .permutation(in_n)
         .count do |order|
           diff = 0
           possible = true
           order.each do |i|
             l, r = in_lr[i]
             if diff > r
               possible = false
               break
             elsif diff < l
               diff = l
             end
           end
           possible
         end
     )