from itertools import permutations
N = int(input())
LR = [list(map(int,input().split())) for _ in range(N)]
ans = 0
for X in permutations(range(N),N):
    dummy = 0
    nowl = 0
    for x in X:
        l,r = LR[x]
        if nowl > r:
            break
        nowl = max(nowl,l)
    else:
        ans += 1
print(ans)