#include <fstream>
#include <queue>
#include <vector>
#include <string>
#include <cstdlib>

using namespace std;
ifstream fin("sate.in");
ofstream fout("sate.out");
const int NMAX = 30000;
long long viz[NMAX+5];
struct muchie
{
   int nod, cost;
};
vector <muchie> G[NMAX+5];
queue <int>q;
string sir;
void bfs(int nod, int finish)
{
   q.push(nod);
   muchie aux;
   viz[nod] =0;
   while(!q.empty())
   {
      nod = q.front();
      q.pop();
      for(int i =0; i<G[nod].size();i++)
      {
         aux = G[nod][i];
         if(viz[aux.nod] == -1)
         {
            viz[aux.nod] = viz[nod] + aux.cost;
            if(aux.nod == finish)
               return;
            q.push(aux.nod);

         }
      }
   }
}
int main()
{
   int n, m, x, y, c, start, finish, i;
   muchie aux;
   fin>>n>>m>>start>>finish;
   while(m--)
   {
       int u,v,w;
       fin>>u>>v>>w;
       G[u].push_back({v,w});
       G[v].push_back({u,-w});
   }
   bfs(start, finish);
   fout<<viz[finish]<<"\n";
    return 0;
}