from heapq import heappop, heappush, heapify
from bisect import bisect
#from sortedcontainers import SortedList
from itertools import permutations
from collections import deque, defaultdict
from math import floor, ceil, isqrt, comb
from sys import stdin, setrecursionlimit
#setrecursionlimit(10**7)
intin = lambda: int(stdin.readline())
strin = lambda: stdin.readline().rstrip()
listin = lambda: list(map(int, stdin.readline().split()))
tuplein = lambda m: [tuple(map(lambda x: int(x) if x.isdigit() or (len(x) > 1 and x[0] == "-" and x[1:].isdigit()) else x, stdin.readline().split())) for _ in range(m)]
gridin = lambda m: [list(map(int, stdin.readline().split())) for _ in range(m)]
strgridin = lambda h: [stdin.readline().rstrip() for _ in range(h)]
mapin = lambda: map(int, stdin.readline().split())
N = intin()
LR = tuplein(N)
ans = 0
for ls in permutations(range(N)):
    prev = 0
    isok = True
    for i in ls:
        if prev <= LR[i][1]:
            prev = max(prev, LR[i][0])
        else:
            isok = False
            break
    if isok:
        ans += 1
print(ans)