from itertools import permutations

N = int(input())
xs = []
for _ in range(N):
    L, R = map(int, input().split())
    xs.append((L, R))


def is_valid(ps):
    p = xs[ps[0]][0]
    for i in range(1, N):
        l, r = xs[ps[i]]
        if p > r: return False
        p = max(p, l)

    return True


ans = 0
for ps in permutations(range(N)):
    if is_valid(ps):
        ans += 1

print(ans)