from itertools import permutations
N = int(input())
D = [list(map(int,input().split())) for _ in range(N)]

Ans = 0
for P in permutations(range(N)):
    diff = 0
    ok = True
    for pi in P:
        if D[pi][1] < diff:
            ok = False
        else:
            diff = max(diff,D[pi][0])
    
    if ok:
        Ans += 1

print(Ans)