import sys
from collections import *
from functools import cache, partial
from itertools import *
from pprint import pprint
from typing import Any, Final

try:
    from icecream import ic
except ImportError:  # Graceful fallback if IceCream isn't installed.
    ic = lambda *a: None if not a else (a[0] if len(a) == 1 else a)  # noqa
debug = partial(print, file=sys.stderr)
dpprint = partial(pprint, stream=sys.stderr)
sys.setrecursionlimit(10 ** 6)
MOD=998244353

N: Final = int(input())
LR = []
for _ in range(N):
    L, R = map(int, input().split())
    LR.append((L, R))
ic(LR)

ans = 0
for P in permutations(range(N)):
    # ic(P)
    is_ok = True
    for pi, pj in zip(P, P[1:]):
        # ic(pi, pj)
        if LR[pi][0] > LR[pj][1]:
            is_ok = False
    if is_ok:
        # ic([p + 1 for p in P])
        ans += 1

print(ans)