from collections import deque, defaultdict, Counter
from bisect import bisect_left, bisect_right
from itertools import permutations, combinations
from heapq import heappop, heappush
import math, sys
# input = sys.stdin.readline
_int = lambda x: int(x)-1
MOD = 998244353 #10**9+7
INF = 1<<60
Yes, No = "Yes", "No"

N = int(input())
LR = []
for _ in range(N):
    l, r = map(int, input().split())
    LR.append((l, r))

ans = 0
for p in permutations(range(N)):
    x = 0
    for i in range(N):
        l, r = LR[p[i]]
        if x > r: break
        x = max(x, l)
    else:
        ans += 1

print(ans)