from itertools import permutations

n = int(input())
lr = [tuple(map(int, input().split())) for _ in range(n)]
ans = 0
for perm in permutations(lr):
    ok = True
    crr_diff = 0
    for i in range(n):
        if crr_diff > perm[i][1]:
            ok = False
            break
        crr_diff = max(crr_diff, perm[i][0])

    ans += ok

print(ans)