from itertools import permutations

n = int(input())
import sys
mp = map(int, sys.stdin.read().split())
lr = list(zip(mp,mp))

ans = 0
for perm in permutations([i for i in range(n)]):
    val = -1
    flag = True
    for id in perm:
        l,r = lr[id]

        if val <= l:
            val = l
            continue

        if val <= r:
            continue

        flag = False
        break

    if flag:
        ans += 1

print(ans)