#include <fstream>
#include <vector>
#include <queue>
using namespace std;
ifstream fin("sate.in");
ofstream fout("sate.out");
int n,m,X,Y,d[30001];
vector< pair<int,int> >h[30001];
queue<int>q;

int main()
{
    fin>>n>>m>>X>>Y;
    for(int i=1;i<=m;i++)
    {
        int x,y,dist;
        fin>>x>>y>>dist;
        h[x].push_back({y,dist});
        h[y].push_back({x,-dist});
    }

    for(int i=1;i<=n;i++)
        d[i]=-1;
    d[X]=0;
    q.push(X);

    while(!q.empty())
    {
        int x=q.front();
        q.pop();
        for(int i=0;i<h[x].size();i++)
        {
            int j=h[x][i].first;
            int dist=h[x][i].second;
            if(d[j]==-1)
            {
                d[j]=d[x]+dist;
                q.push(j);
                if(d[Y]>=0)
                {
                    fout<<d[Y];
                    return 0;
                }
            }
        }

    }

    return 0;
}