#include <fstream>
#include <vector>
#include <queue>
#define pb push_back
#define mp make_pair
using namespace std;
ifstream f("sate.in");
ofstream h("sate.out");
vector < pair <int, int> > g[30001];
queue <int> q;
int n,m,a,b,x,y,d,i,t[30001],s,way[30001];
bool sel[30001];
void bf (int x)
{
    int nod,p,i,nurm,c;
    q.push(x); sel[x]=true;way[x]=0;
    while (q.back()!=b)
    {
       nod=q.back();
       for (i=0;i<g[nod].size();i++)
        {
           nurm=g[nod][i].first;
           c=g[nod][i].second;
           if ( !sel[nurm])
            {
              q.push(nurm);
              sel[nurm]=true;
              way[nurm]=way[nod]+c;
            }
        }
     }
}
int main()
{
vector <int> :: iterator it;
f>>n>>m>>a>>b;
     for (i=1;i<=m;i++)
     {
         f>>x>>y>>d;
         g[x].pb(mp(y,d));
         g[y].pb(mp(x,-d));
     }
bf(a);
h<<way[b]<<'\n';
return 0;
}