#include <fstream>
#include <vector>
#include <utility>
#include <queue>
#define NMAX 30001
using namespace std;

ifstream fin("sate.in");
ofstream fout("sate.out");
long long viz[NMAX+5];
int n;
struct muchie
{
   int nod, cost;
};
vector <muchie> g[NMAX+5];
queue <int>q;
string sir;
int 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 viz[finish];
            q.push(aux.nod);

         }
      }
   }
}
int main() {
    int m,source,dest;
    fin>>n>>m>>source>>dest;

    while(m>0)
    {
        int u,v,w;
        fin>>u>>v>>w;
        g[u].push_back({v,w});
        g[v].push_back({u,-w});
        --m;
    }

    fout<<BFS(source,dest)+1;
    //fout<<res;

	return 0;
}