题目如下：

Til the Cows Come Home
Time Limit: 1000MS        Memory Limit: 65536K
Total Submissions: 27726        Accepted: 9353

Description
Bessie is out in the field and wants to get back to the barn to get as much sleep as possible before Farmer John wakes her

for the morning milking. Bessie needs her beauty sleep, so she wants to get back as quickly as possible.

Farmer John‘s field has N (2 <= N <= 1000) landmarks in it, uniquely numbered 1..N. Landmark 1 is the barn; the apple tree

grove in which Bessie stands all day is landmark N. Cows travel in the field using T (1 <= T <= 2000) bidirectional cow-

trails of various lengths between the landmarks. Bessie is not confident of her navigation ability, so she always stays on

a trail from its start to its end once she starts it.

Given the trails between the landmarks, determine the minimum distance Bessie must walk to get back to the barn. It is

guaranteed that some such route exists.

Input
* Line 1: Two integers: T and N

* Lines 2..T+1: Each line describes a trail as three space-separated integers. The first two integers are the landmarks

between which the trail travels. The third integer is the length of the trail, range 1..100.

Output
* Line 1: A single integer, the minimum distance that Bessie must travel to get from landmark N to landmark 1.

Sample Input

5 5
1 2 20
2 3 30
3 4 20
4 5 20
1 5 100

Sample Output

90

Hint
INPUT DETAILS:

There are five landmarks.

OUTPUT DETAILS:

Bessie can get home by following trails 4, 3, 2, and 1.

求最短路，经典的dijkstra算法直接解决了，分别用邻接表和邻接矩阵存图。

邻接矩阵版：