#include <iostream>
#include <fstream>
#include <cstring>
#include <vector>
#include <queue>
#define mp make_pair

using namespace std;

string filename = "sate";

ifstream fin(filename + ".in");
ofstream fout(filename + ".out");

const int NMAX = 30000;

int dist[NMAX + 5]; /// dist de la x la satul respectiv
vector<pair<int,int>> adj[NMAX + 5];

int main()
{
    int n,m,x,y;
    fin>>n>>m>>x>>y;
    for(int i=1;i<=m;i++)
    {
        int x,y,d;
        fin>>x>>y>>d;
        adj[x].push_back(mp(y, d));
        adj[y].push_back(mp(x,-d));
    }
    for(int i=1;i<=n;i++)
        dist[i] = -1;

    queue<int> q;
    q.push(x);
    dist[x] = 0;
    while(!q.empty())
    {
        int nod = q.front();

        for(auto it : adj[nod])
        {
            if(dist[it.first] == -1)
            {
                dist[it.first] = dist[nod] + it.second;
                q.push(it.first);
            }
        }
        q.pop();
    }
    fout<<dist[y];
    return 0;
}