# P2886 [USACO07NOV]牛继电器Cow Relays

• 479通过
• 989提交
• 题目提供者 FarmerJohn2
• 评测方式 云端评测
• 标签 动态规划,动规,dp 最短路 矩阵乘法 USACO 2007 高性能
• 难度 提高+/省选-
• 时空限制 1000ms / 128MB
• 提示：收藏到任务计划后，可在首页查看。

## 题目描述

For their physical fitness program, N (2 ≤ N ≤ 1,000,000) cows have decided to run a relay race using the T (2 ≤ T ≤ 100) cow trails throughout the pasture.

Each trail connects two different intersections (1 ≤ I1i ≤ 1,000; 1 ≤ I2i ≤ 1,000), each of which is the termination for at least two trails. The cows know the lengthi of each trail (1 ≤ lengthi  ≤ 1,000), the two intersections the trail connects, and they know that no two intersections are directly connected by two different trails. The trails form a structure known mathematically as a graph.

To run the relay, the N cows position themselves at various intersections (some intersections might have more than one cow). They must position themselves properly so that they can hand off the baton cow-by-cow and end up at the proper finishing place.

Write a program to help position the cows. Find the shortest path that connects the starting intersection (S) and the ending intersection (E) and traverses exactly N cow trails.

给出一张无向连通图，求S到E经过k条边的最短路。

## 输入输出格式

输入格式：

* Line 1: Four space-separated integers: N, T, S, and E

* Lines 2..T+1: Line i+1 describes trail i with three space-separated integers: lengthi , I1i , and I2i

输出格式：

* Line 1: A single integer that is the shortest distance from intersection S to intersection E that traverses exactly N cow trails.

## 输入输出样例

输入样例#1： 复制
2 6 6 4
11 4 6
4 4 8
8 4 9
6 6 8
2 6 9
3 8 9
输出样例#1： 复制
10
提示
标程仅供做题后或实在无思路时参考。
请自觉、自律地使用该功能并请对自己的学习负责。
如果发现恶意抄袭标程，将按照I类违反进行处理。