from itertools import permutations

N = int(input())
intervals = [tuple(map(int, input().split())) for _ in range(N)]

def ok_order(order):
    cur = -10**18
    for idx in order:
        L, R = intervals[idx]
        if R < cur:
            return False
        cur = max(cur, L)
    return True

ans = 0
for perm in permutations(range(N)):
    if ok_order(perm):
        ans += 1

print(ans)