N = int(input())
l = []
r = []
for i in range(N):
    L,R = map(int, input().split())
    l.append(L)
    r.append(R)
from itertools import permutations
ans = 0
for pt in permutations([i for i in range(N)]):
    now = 0
    flag = True
    for _ in range(len(pt)):
        L,R = l[pt[_]],r[pt[_]]
        if now < L:
            now = L
        elif L <= now < R:
            continue
        else:
            flag = False
   # print(now)
    if flag:
        ans += 1
print(ans)