N=int(input())
P=[]
for i in range(N):
    L,R=map(int,input().split())
    P.append([L,R])
ans=0
import itertools
for i in itertools.permutations(range(N)):
    mini=-1
    ng=False
    for j in range(N):
        if P[i[j]][1]<mini:
            ng=True
            break
        else:
            mini=max(mini,P[i[j]][0])
    if not ng:
        ans+=1
print(ans)