import itertools
import sys
input = sys.stdin.readline
N = int(input())
LR = [list(map(int, input().split())) for _ in range(N)]
ans = 0
for P in itertools.permutations(range(N)):
    pre = 0
    flag = True
    for i in P:
        l, r = LR[i]
        if r<pre:
            flag = False
            break
        pre = max(l, pre)
    if flag:
        ans += 1
print(ans)