def ip():return int(input())
def mp():return map(int, input().split())
def lmp():return list(map(int, input().split()))
# 岩プロコン2 D No.3262 水色コーダーさん、その問題d問題ですよ？（1<=d<=N）
from itertools import permutations
N = ip()
LR = [lmp() for _ in range(N)]
ans = 0
for p in permutations(range(N)):
  ok = True
  cur = LR[p[0]][0]
  for i in range(1, N):
    l, r = LR[p[i]]
    if r < cur:
      ok = False
      break
    cur = max(cur, l)
  if ok:
    ans += 1
print(ans)