import itertools

N = int(input())
junretu = itertools.permutations(range(N),N)

lst = []
for i in range(N):
    a,b = map(int,input().split())
    lst.append((a,b))

cnt = 0
for j in junretu:
    now = 0
    check = True
    for k in range(N):
        if now > lst[j[k]][1]:
            check = False
            break
        else:
            now = max(now,lst[j[k]][0])
    if check:
        #print(j)
        cnt += 1

print(cnt)