n = int(input())
from itertools import permutations as P
LR = [tuple(map(int, input().split())) for _ in range(n)]
ans = 0
for per in P(range(n), n):
    l = 0
    for i in range(n):
        if l > LR[per[i]][1]:
            break
        l = max(l, LR[per[i]][0])
    else:
        ans += 1
print(ans)