from itertools import groupby, accumulate, combinations, permutations, product, combinations_with_replacement

n = int(input().strip())
L = []
R = []

for i in range(n):
    l,r = map(int, input().split())
    L.append(l)
    R.append(r)

cnt = 0
for per in permutations(range(n)):
    flag = True
    now = 0
    for num in per:
        now = max(now, L[num])
        if now > R[num]:
            flag = False
            break
    if flag:
        cnt += 1

print(cnt)