from itertools import permutations

n = int(input())

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

ans = 0
for v in permutations(range(n)):
    p = -1
    ok = True
    for i in v:
        l, r = data[i]
        if r < p:
            ok = False
        p = max(p, l)
    if ok:
        ans += 1

print(ans)