N = gets.to_i
LR = Array.new(N) { gets.split.map(&:to_i) }

cnt = 0
LR.permutation(N).each do |perm|
  ok = true
  min = 0
  (N - 1).times do |i|
    l1, r1 = perm[i]
    l2, r2 = perm[i + 1]
    if r2 < min
      ok = false
      break
    end
    unless l1 <= r2 && l2 <= r1 || r1 <= l2
      ok = false
      break
    end
    min = [min, l1, l2].max
  end
  next unless ok
  cnt += 1
end

puts cnt