老規矩，先放模板，有時間放圖解

#include <map>
#include <queue>
#include <cstdlib>
#include <cmath>
#include <cstdio>
#include <string>
#include <cstring>
#include <fstream>
#include <iostream>
#include <sstream>
#include <algorithm>
#define lowbit(a) (a&(-a))
 

#define _mid(a,b) ((a+b)/2)
#define _mem(a,b) memset(a,0,(b+3)<<2)
#define fori(a) for(int i=0;i<a;i++)
#define forj(a) for(int j=0;j<a;j++)
#define ifor(a) for(int i=1;i<=a;i++)
#define jfor(a) for(int j=1;j<=a;j++)
#define mem(a,b) memset(a,b,sizeof(a))
#define IO do{\
    ios::sync_with_stdio(false);\
    cin.tie(0);\
    cout.tie(0);}while(0)
 

#define mp(a,b) make_pair(a,b)
#define pb(a) push_back(a)
#define debug(a) cout <<(a) << endl

using namespace std;
typedef long long ll;
const int maxn =  1e3+5;
const int INF = 0x3f3f3f3f;

int  s,t,k;
bool vis[maxn];
int dis[maxn];
struct node{
    int v,c;
    node(int _v=0,int _c=0) :
 
 v(_v),c(_c) {};    //構造
    node(){};
    bool operator < (const node & buf) const{		
        return c+ dis[v]  > buf.c + dis[buf.v];
    }
};

struct edge{
    int v,cost;
    edge(int _v=0,int _c=0) : v(_v),cost(_c){};
};

vector <edge> e[maxn],reve[maxn];      //反向存圖
priority_queue<node> q;
void dijkstra(int n,int s){		//dijkstra+佇列優化
    mem(vis,false);
    mem(dis,0x3f);
    while(!q.empty()) q.pop();
    dis[s] = 0;
    q.push(node(s,0));
    while(!q.empty()){
        node tmp = q.top();
        q.pop();
        int u = tmp.v; 
        if(vis[u])
            continue;
        vis[u] = true;
        fori(e[u].size()){
            int v = e[u][i].v;
            int cost = e[u][i].cost;
            if(!vis[v] && dis[v] > dis[u] + cost){
                dis[v] = dis[u] + cost;
                q.push(node(v,dis[v]));
            }
        }
    }
}


int aStar(int s){
    while(!q.empty()) q.pop();
    q.push(node(s,0));
    k--;          
    while(!q.empty()){
        node pre = q.top();
        q.pop();
        int u = pre.v;
        if(u == t){
            if(k)   k--;
            else    return pre.c;
        }
        fori(reve[u].size()){
            int v = reve[u][i].v;
            int c = reve[u][i].cost;
            q.push(node(v,pre.c+c));
        }
    }
    return -1;
}

void addedge(int u,int v,int w){
    reve[u].pb(edge(v,w));
    e[v].pb(edge(u,w));
}
int main() {
    IO;
    int n,m,u,v,w;
    while(cin>>n >> m){
        fori(n+2){
            e[i].clear();
            reve[i].clear();
        }
        fori(m){
            cin >> u >> v >> w;
            addedge(u,v,w);
        }
        cin >> s >> t >> k;
        dijkstra(n,t);
        if(dis[s] == INF)
            cout << -1 << endl;
        else{
            if(s == t)
                k ++;  ///如果起點==終點不能算上dis = 0 的這一點
            cout << aStar(s) << endl;;
        }
    }
    return 0;
}