#include <iostream>
#include <fstream>
#include <vector>
#include <queue>
using namespace std;

long n,m,x,y,q,w,c,viz[30005],cost[30005];
struct gr
{
    long r,c;
}punct,nod;
vector<gr>a[30005];
queue<gr> coada;
void bfs()
{

   punct.r = x;
   punct.c = 0;
   coada.push(punct);
   viz[x]=1;
   cost[x]=1;
   while(!coada.empty() && viz[y]==0)
   {

       nod = coada.front();
       coada.pop();
        viz[nod.r]=1;

       for(int i=0;i<a[nod.r].size();i++)
       {
           if(viz[a[nod.r][i].r]==0)
           {
               viz[a[nod.r][i].r]=1;
            if(cost[a[nod.r][i].r]==0)
                cost[a[nod.r][i].r] = cost[nod.r] + a[nod.r][i].c;
               coada.push(a[nod.r][i]);
           }
       }
              //cost = cost - nod.c;
   }

}

int main()
{
   ifstream in("sate.in");
   ofstream out("sate.out");
   in >> n >> m >> x >> y;
   for(int i=0;i<m;i++)
   {
       in >> q >> w >> c;
       punct.r = w;
       punct.c = c;
       a[q].push_back(punct);
       punct.r = q;
       punct.c = -c;
       a[w].push_back(punct);
   }

   bfs();
      out << cost[y]-1;
    return 0;
}