from itertools import permutations
N = int(input())
probs = []
for _ in range(N):
  L, R = map(int, input().split())
  probs.append((L, R))

cnt = 0
isValid = True
for perm in permutations(probs):
  x = -float("inf")
  for L, R in perm:
    x = max(L, x)
    if x > R:
      isValid = False
      break
  if isValid:
    cnt += 1
  else:
    isValid = True

print(cnt)