from itertools import permutations
N = int(input())
cont = []
for _ in range(N):
    l,r = list(map(int,input().split()))
    cont.append((l,r))

ans = 0
for i in permutations(cont):
    now = 0
    flag = True
    for l,r in i:
        if(r < now):
            flag = False
            break
        
        if(now < l):now = l
    ans += flag
print(ans)