#include <bits/stdc++.h>

using namespace std;

ifstream fin("sate.in");
ofstream fout("sate.out");
///          xi  d
vector< pair < int,int > >h[30001];
queue<int>q;
///   d[i]=distanta de la x la i

int d[30001];

int main()
{
    int i,m,x,y,dist,X,Y,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;
}