from itertools import permutations

N = int(input())
LR = [list(map(int, input().split())) for _ in range(N)]

ans = 0
for perm in permutations(range(N)):
  mxl = -1
  for i in perm:
    l, r = LR[i]
    if mxl <= r:
      mxl = max(mxl, l)
    else:
      break
  else:
    ans += 1

print(ans)