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

using namespace std;

struct Nr
{
    int sat,cost;
};
vector <Nr> v[30001];
queue <int> q;

int d[30001],X,Y;
unsigned int N;

inline void Read()
{
    int x,y,dist;
    unsigned int i,M;
    Nr w;
    ifstream fin("sate.in");
    fin>>N>>M>>X>>Y;
    for(i=1;i<=M;i++)
    {
        fin>>x>>y>>dist;
        w.sat=x;w.cost=dist;
        v[y].push_back(w);
        w.sat=y;
        v[x].push_back(w);
    }
    fin.close();
}

inline void Solve()
{
    int j,k,c;
    unsigned int i;
    q.push(X);
    for(i=1;i<=N;i++)
        d[i]=-1;
    d[X]=0;
    while(!q.empty())
    {
        j=q.front();
        q.pop();
        for(i=0;i<v[j].size();i++)
        {
            k=v[j][i].sat;
            c=v[j][i].cost;
            if(d[k]<0)
            {
                if(j<k)
                    d[k]=d[j]+c;
                else
                    d[k]=d[j]-c;
                q.push(k);
                if(d[Y]>=0)
                   return ;
            }
        }
    }
}

int main()
{
    Read();
    Solve();
    ofstream fout("sate.out");
    fout<<d[Y]<<"\n";
    fout.close();
    return 0;
}