#include <fstream>
#include <vector>
#include <queue>
#define IN "sate.in"
#define OUT "sate.out"
#define NMAX 30005

using namespace std;

ifstream in(IN);
ofstream out(OUT);

vector < pair <int, int> > G[NMAX];
queue <int> Q;
int n, X, Y, d[NMAX];
bool Use[NMAX];

inline int BFS()
{
    int i, nod;
    Q.push(X);
    Use[X]=true;
    while(!Q.empty())
    {
        nod=Q.front();
        Q.pop();
        for(i=0; i<G[nod].size(); ++i)
            if(!Use[G[nod][i].first])
            {
                Use[G[nod][i].first]=true;
                if(nod > G[nod][i].first)
                    d[G[nod][i].first] = d[nod] + G[nod][i].second;
                else
                    d[G[nod][i].first] = d[nod] - G[nod][i].second;
                if(G[nod][i].first == Y)
                    return -d[Y];
                else Q.push(G[nod][i].first);
            }
    }
    return -1;
}

int main()
{
    int m, i, a, b, c;
    in>>n>>m>>X>>Y;
    while(m--)
    {
        in>>a>>b>>c;
        G[a].push_back(make_pair(b, c));
        G[b].push_back(make_pair(a, c));
    }
    out<<BFS()<<'\n';
    in.close();
    out.close();
    return 0;
}