import sys, time, random, heapq, math, itertools, copy
from collections import deque, Counter, defaultdict
from bisect import bisect, bisect_left, bisect_right
import heapq as hq
from functools import cache, cmp_to_key
def debug(*x):print('debug:',*x, file=sys.stderr)
sys.setrecursionlimit(300000)
input = lambda: sys.stdin.readline().rstrip()
ii = lambda: int(input())
mi = lambda: map(int, input().split())
li = lambda: list(mi())
inf = 2 ** 61 - 1
mod = 998244353
dir = [(0, 1), (0, -1), (1, 0), (-1, 0)]

def dfs(n, diff):
    global visited, ans
    if n == N:
        ans += 1
        return 0
    for i in range(N):
        if not visited[i] and R[i]>=diff:
            visited[i] = True
            temp = max(diff, L[i])
            dfs(n+1, temp)
            visited[i] = False


N = ii()
L = [0] * N; R = [0] * N
for i in range(N):
    L[i], R[i] = mi()
ans=0
visited = [False] * N
dfs(0,0)
print(ans)