N=int(input())

P=[list(map(int,input().split())) for i in range(N)]

from itertools import permutations

L=list(permutations(range(N)))

ANS=0

for X in L:
    now=0

    for x in X:
        l,r=P[x]

        if r<now:
            break
        else:
            now=max(now,l)
    else:
        ANS+=1

print(ANS)