#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;
    cin>>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())
    {
        if(d[Y]>=0)
        {
            cout<<d[Y];
            return 0;
        }
    }


    return 0;
}