from itertools import permutations


N = int(input())
LR = [tuple(map(int, input().split())) for _ in range(N)]
ans = 0
for perm in permutations(range(N)):
    pv = 0
    for i in perm:
        l, r = LR[i]
        if pv <= r:
            pv = max(pv, l)
        else:
            break
    else:
        ans += 1
print(ans)