#include <bits/stdc++.h>

using namespace std;
ifstream fin ("sate.in");
ofstream fout ("sate.out");
const int NMAX=3*1e4+5;
int n,m,x,y,dist[NMAX];
vector< pair<int,int> > G[NMAX];
queue <int> q;
void citire()
{
    fin>>n>>m>>x>>y;
    while(m--)
    {
        int a,b,c;
        fin>>a>>b>>c;
        G[a].push_back({b,c});
        G[b].push_back({a,-c});
    }
}
void bfs()
{
    q.push(x);
    dist[x]=1;
    if(x==y) return ;
    while(!q.empty())
    {
        int nods=q.front();
        q.pop();
        for(int i=0;i<G[nods].size();i++)
        {
             if(!dist[G[nods][i].first])
            {
                dist[G[nods][i].first]=dist[nods]+G[nods][i].second;
                q.push(G[nods][i].first);
             if(G[nods][i].first==y) break;

            }
        }
    }
}
int main()
{
    citire();
    bfs();
    fout<<dist[y]-1;
    return 0;
}