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

#define d first
#define x second.first
#define y second.second
using namespace std;

ifstream fin("sate.in");
ofstream fout("sate.out");

const int NMAX = 30002;

int N,M,X,Y;
int a,b,c;
vector <pair<int,pair<int,int> > > V;
void Read()
{
    fin>>N>>M>>X>>Y;
    for(int i=1; i<=M; ++i)
    {
        fin>>a>>b>>c;
        V.push_back(make_pair(c,make_pair(a,b)));
    }
}
void Do()
{
    int dist[30005];
    dist[X]=1;
    while(!dist[Y])
    {
        for(int i=0; i<V.size(); ++i)
        {
            if(dist[V[i].x] && !dist[V[i].y])
                dist[V[i].y]=dist[V[i].x]+V[i].d;

            if(!dist[V[i].x] && dist[V[i].y])
                dist[V[i].x]=dist[V[i].y]-V[i].d;
        }
    }
    fout<<dist[Y]-1;
}
int main()
{
    Read();
    Do();
    return 0;
}