N=int(input())
A=[]
for i in range(N):
  l,r=map(int,input().split())
  A.append((l,r))
from itertools import permutations
L=[i for i in range(N)]
result=0
for P in permutations(L):
  x=0
  ans=True
  for i in range(N):
    pos=P[i]
    l,r=A[pos][0],A[pos][1]
    if x<=l:
      x=l
    elif l<=x<=r:
      x+=0
    else:
      ans=False
  if ans==True:
    result+=1
print(result)