内存限制：64 MiB 时间限制：5 Sec

The cows have been sent on a mission through space to acquire a new
milking machine for their barn. They are flying through a cluster
of stars containing N (1 <= N <= 50,000) planets, each with a trading
post.
The cows have determined which of K (1 <= K <= 1,000) types of
objects (numbered 1..K) each planet in the cluster desires, and
which products they have to trade. No planet has developed currency,
so they work under the barter system: all trades consist of each
party trading exactly one object (presumably of different types).
The cows start from Earth with a canister of high quality hay (item
1), and they desire a new milking machine (item K). Help them find
the best way to make a series of trades at the planets in the cluster
to get item K. If this task is impossible, output -1.

* Line 1: Two space-separated integers, N and K.
* Lines 2..N+1: Line i+1 contains two space-separated integers, a_i
and b_i respectively, that are planet i's trading trading
products. The planet will give item b_i in order to receive
item a_i.

* Line 1: One more than the minimum number of trades to get the
milking machine which is item K (or -1 if the cows cannot
obtain item K).

```
6 5 //6个星球,希望得到5,开始时你手中有1号货物.
```

1 3 //1号星球,希望得到1号货物,将给你3号货物

3 2

2 3

3 1

2 5

5 4

```
4
```

OUTPUT DETAILS:

The cows possess 4 objects in total: first they trade object 1 for

object 3, then object 3 for object 2, then object 2 for object 5.