#include <bits/stdc++.h>
using namespace std;
ifstream in("sate.in");
ofstream out("sate.out");
int n,m,x,y,dis[30005];
vector <pair<int,int>> sate[30005];
queue <int> q;
int main()
{int i,a,b,k,d,c;
    in>>n>>m>>x>>y;
    for (i=1;i<=m;i++)
    {
        in>>a>>b>>c;
        sate[a].push_back({b,c});
        sate[b].push_back({a,-1*c});
    }
    for (i=1;i<=n;i++)dis[i]=-1;
   q.push(x);
   dis[x]=0;
    while (!q.empty())
    {
        k=q.front();
        q.pop();
        for (auto w : sate[k])
        {
            i=w.first;
            d=w.second;
            if (dis[i]==-1)
            {
                dis[i]=dis[k]+d;
                if (i==y)
                {
                    out<<dis[y]<<"\n";return 0;
                }
                q.push({i});

            }
        }

    }
    out<<dis[y]<<"\n";
    out.close();
    in.close();
    return 0;
}