from itertools import permutations 

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

P = permutations(list(range(N)))

ans = 0
Lmax = 0
for p in P:
    flag = True
    Lmax = 0
    for i in range(N):
        L, R = LR[p[i]]
        if R < Lmax:
            flag = False
        Lmax = max(Lmax,L)

    if flag:
        ans += 1

print(ans)