N = int(input())

LR = []

for i in range(N):
    L,R = map(int,input().split())
    LR.append((L,R))

import itertools

ans = 0
for p in itertools.permutations([i for i in range(N)]):
    
    x = 0

    for v in p:
        L,R = LR[v]
        if R < x:
            break
        x = max(L,x)
    else:
        ans += 1

print (ans)