用最短路径算法算出特定两点之间的距离
输入：直接读取net.in文件
第一行为四个整数N,M,S,T,每两个整数之间用一个空格隔开，分别表示网络的节点（编号为1到N），网络线段数，以及起点终点编号
接下来M行，每行三个整数U,V,D，每个整数之间用空格隔开，表示节点U和V之间有一条线路相连，距离是D，（两个节点可能有多条线路）
输出：直接写入到net.out文件，输出只有一行，表示从S点到T点的最短距离（如果不通则输出-1）　（N<=100，M<=N*N,D<=1000）(Use the shortest path algorithm to calculate the distance between two specific two points
Input: read the net.in file directly
The first behavior is four integers N, M, S and T. Every two integers are separated by a space, representing the node of the network (number 1 to N), the number of network segments, and the origin and destination number.
Next, M row, each row three integers U, V, D, each integer is separated by spaces, indicating that there is a line between nodes U and V, the distance is D, (there may be multiple lines in two nodes).
Output: write directly to net.out file, output only one line, which indicates the shortest distance from S point to T point (if it doesn't connect, output -1) (N<=100, M<=N*N, D<=1000).

Floyd-Warshall algorithm)