import itertools

N = int(input())

A = []
for i in range(N):
	A.append(i)

T = []
for i in range(N):
	L, R = list(map(int,input().split()))
	T.append((L, R))
	
ans = 0
for per in itertools.permutations(A):
	now = 0
	ok = 1
	for i in range(N):
		L, R = T[per[i]]
		if R < now:
			ok = 0
			break
		elif now < L:
			now = L
		else:
			continue
	ans += ok
print(ans)