n=int(input())
a=[tuple(map(int,input().split())) for i in range(n)]
from itertools import permutations
c=0
for p in permutations(a):
  now=0
  for l,r in p:
    if now<l:
      now=l
    elif now+1<=r:
      now=now+1
    else:
      break
  else:
    c+=1
print(c)