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

using namespace std;

int n,m,x,y;
vector <pair <int,int> > v[30005];
int use[30005];
queue<int> q;

void citire()
{
    ifstream fin("sate.in");
    fin>>n>>m>>x>>y;

    for(int i=1;i<=m;i++)
    {
        int j,k,l;
        fin>>j>>k>>l;
        v[j].push_back(make_pair(k,l));
        v[k].push_back(make_pair(j,-l));
    }
}
void BFS()
{
    int nod,vecin;
    memset(use,-1,sizeof(use));
    use[x]=0;
    q.push(x);
    while(!q.empty())
    {
        nod=q.front();q.pop();
        for(int i=0;i<v[nod].size();i++)
        {
            vecin=v[nod][i].first;
            if(use[vecin]==-1)
            {
                use[vecin]=use[nod]+v[nod][i].second;
                q.push(vecin);
            }
        }

    }

    ofstream fout("sate.out");
    fout<<use[y];

}
/*
void afis()
{
    for(int i=1;i<=n;i++)
    {
        cout<<i<<":";
        for(int j=0;j<v[i].size();j++)
            cout << v[i][j].first << ","<<v[i][j].second<<"  ";
        cout<<endl;
    }
    for(int i=1;i<=n;i++)
        cout<<i<<":"<<use[i]<<" ";
}*/
int main()
{
    citire();
BFS();
}