from itertools import permutations,combinations,groupby

n = int(input())
lr = [list(map(int,input().split())) for _ in range(n)]
l,r = [list(i) for i in zip(*lr)]

ans = 0
for p in permutations(range(n)):
    prev = 0
    for i in range(n):
        if r[p[i]] < prev:
            break
        prev = max(prev, l[p[i]])
    else:
        ans += 1
print(ans)