#include <bits/stdc++.h>
using namespace std;
ifstream fin("sate.in");
ofstream fout("sate.out");
vector <pair<int,int> >h[30001];
queue <int>q;
int d[30001],X,Y;
int main()
{
    int i,m,x,y,dist,n,j;
    fin>>n>>m>>X>>Y;
    for(i=1;i<=m;i++)
    {
        fin>>x>>y>>dist;
        h[x].push_back({y,dist});
        h[y].push_back({x,-dist});
    }
    for(i=1;i<=n;i++)
    d[i]=-1;
d[X]=0;
    q.push(X);
    while(!q.empty())
    {
        x=q.front();
        q.pop();
        for(auto w: h[x])
        {
            j=w.first;
            dist=w.second;
            if(d[j]==-1)
            {
                d[j]=d[x]+dist;
                q.push(j);
            if(d[Y]>=0)
        {
            fout<<d[Y];
            return 0;
        }
            }
        }

    }


    return 0;
}