#include <vector>
#include <fstream>
#include <queue>
using namespace std;
ifstream f("sate.in");
ofstream g("sate.out");

#define Nmax 30005
vector < pair <int,int> > G[Nmax];
queue <int>Q;
int n,m,alfa,omega;
long long d[Nmax];
void read()
{
    f>>n>>m>>alfa>>omega;
    for(int i=1;i<=m;i++)
    {
        int x,y,c;
        f>>x>>y>>c;
        G[x].push_back( make_pair(y,c));
        G[y].push_back( make_pair(x,-c));
    }
     for(int i=1;i<=n;i++)
         d[i]=-1;
     d[alfa]=0;
     Q.push(alfa);
}
void bfs(int nod)
{
    while(!Q.empty())
    {
        nod=Q.front();
        Q.pop();
        for(int i=0;i<G[nod].size();i++)
        {
            int vecin=G[nod][i].first;
            if(d[vecin]==-1)
            {
                d[vecin]=d[nod]+G[nod][i].second;
                if(vecin==omega) break;
                Q.push(vecin);
            }
        }
        if(nod==omega) break;
    }
}
int main()
{
    read();
    bfs(alfa);
    g<<d[omega];
    return 0;
}