n = int(input())
a = [i for i in range(n)]
p = [list(map(int,input().split())) for _ in range(n)]
import itertools
ans = 0
for A in itertools.permutations(a):
    k = 0
    h = True
    for q in A:
        if k > p[q][1]:
            h = False
            break
        k = max(k,p[q][0])
    if h: ans += 1
print(ans)