#include <iostream>
#include <fstream>
#include <vector>
#include <queue>

#define v first
#define c second
#define NMax 30005

using namespace std;

vector < pair <int, int> > G[NMax];
int N, Start, End;
queue <int> Q;
int D[NMax];

void Read ()
{
    ifstream fin ("sate.in");
    int M;
    fin >> N >> M >> Start >> End;
    for (; M>0; --M)
    {
        int X, Y, Z;
        fin >> X >> Y >> Z;
        if (X>Y)
        {
            int Aux=X;
            X=Y;
            Y=Aux;
        }
        G[X].push_back (make_pair (Y, Z));
        G[Y].push_back (make_pair (X, -Z));
    }
    fin.close ();
}

void BFS ()
{
    Q.push (Start);
    while (!Q.empty ())
    {
        int X=Q.front ();
        Q.pop ();
        if (X==End)
        {
            break;
        }
        for (unsigned i=0; i<G[X].size (); ++i)
        {
            int V=G[X][i].v;
            int C=G[X][i].c;
            if (!D[V])
            {
                D[V]=D[X]+C;
                Q.push (V);
            }
        }
    }
}

void Print ()
{
    ofstream fout ("sate.out");
    fout << D[End] << "\n";
    fout.close ();
}

int main()
{
    Read ();
    BFS ();
    Print ();
    return 0;
}