from itertools import permutations
N = int(input())
LR = [tuple(map(int, input().split())) for _ in range(N)]

ans = 0
for pat in permutations([i for i in range(N)]):

    pos = LR[pat[0]][0]
    for i in range(1, N):
        l, r = LR[pat[i]]
        if r < pos:
            break
        pos = max(pos, l)
    else:
        ans += 1

print(ans)