import itertools

def IM(): return map(int, input().split())

N = int(input())
P = []
for i in range(N):
    p = tuple(IM())
    P.append(p)

ans = 0
for p in itertools.permutations(list(range(N))):
    now_mindiff = P[p[0]][0]
    judge = 1
    for j in range(1, N):
        if P[p[j]][1] < now_mindiff:
            judge = 0
            break
        now_mindiff = max(P[p[j]][0], now_mindiff)
    ans += judge

print(ans)