import itertools

N = int(input())
P = [tuple(map(int, input().split())) for _ in range(N)]

ans = 0
for X in itertools.permutations(P):
    L = -1
    for l, r in X:
        if r < L:
            break
        
        L = max(L, l)
    else:
        ans += 1

print(ans)