#include <iostream>
#include <fstream>
#include <vector>
#include <queue>

using namespace std;

ifstream fin("sate.in");
ofstream fout("sate.out");

struct graf{
    int vecin, cost;
};

vector <graf> G[30001];
int n, m, X, Y;
int dist[30001];

void bfs(int nod)
{
    queue <int> que;
    for(int i=1; i<=n; i++)
        dist[i] = -1;

    dist[nod] = 0;
    que.push(nod);

    while(!que.empty())
    {
        int u = que.front();
        que.pop();
        for(int i=0; i<G[u].size(); i++)
        {
            int v = G[u][i].vecin;
            if(dist[v] == -1)
            {
                dist[v] = dist[u] + G[u][i].cost;
                que.push(v);
            }
        }
    }

}

int main()
{
    fin >> n >> m >> X >> Y;
    for(int i=1; i<=m; i++)
    {
        int x, y, c;
        fin >> x >> y >> c;
        G[x].push_back({y, c});
        G[y].push_back({x, -c});
    }

    bfs(X);
    fout << dist[Y];

    return 0;
}