#include <bits/stdc++.h>

using namespace std;

vector< pair <int,int> > h[30001];
queue<int> q;
int d[30001],X,Y;


int main()
{
    int i,m,n,x,y,dist,j;
    cin >> n >> m >> X >> Y;
    for (i=1; i<=m; i++)
    {
        cin >> 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)
        {
            cout << d[Y];
            return 0;
        }
            }
        }

    }






    return 0;


}