from itertools import *

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

ans=0
for p in permutations(range(N)):
    d=0
    for i in p:
        if d>P[i][1]:
            break
        d=max(d,P[i][0])
    else:
        ans+=1

print(ans)