import sys
sys.setrecursionlimit(10**7)
def ii(): return int(input())
def ist(): return input().split()
def mi(d=0): return map(lambda x:int(x)-d,input().split())
def lmi(d=0): return list(map(lambda x:int(x)-d,input().split()))
INF = float("inf")
def answer(s):
  print(s)
  exit()
################################################
n = ii()
lr = [lmi() for _ in range(n)]
visit = [False]*n

def solve(before_min,before_index):
  if sum(visit) == n:
    visit[before_index] = False
    return 1
  r = 0
  for i in range(n):
    if not visit[i]:
      if before_min <= lr[i][1]:
        visit[i] = True
        r += solve(max(before_min,lr[i][0]),i)
  visit[before_index] = False
  return r
print(solve(0,-1))