#include <bits/stdc++.h>

using namespace std;

struct dublu
{
    int nod,cost;
};

vector <dublu> v[30005];
int n,m,X,Y,d[30005];
queue <int> q;

inline void Citire()
{
    int i,x,y,lung;
    dublu w;
    ifstream fin("sate.in");
    fin>>n>>m>>X>>Y;
    for(i=1;i<=m;i++)
    {
        fin>>x>>y>>lung;
        w.nod=y;
        w.cost=lung;
        v[x].push_back(w);
        w.nod=x;
        v[y].push_back(w);
    }
    fin.close();
}

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

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