#include<iostream>
#include<fstream>
#include<vector>
#include<queue>
using namespace std;


vector<vector<pair<int, int>>> adj;
vector<int> viz;
vector<int> dist; // distance from x to temp_dest
int n, m, x, y;


ifstream file_in("sate.in");
ofstream file_out("sate.out");


int BFS(int s){

    viz[s] = 1;
    queue<int> q;

    q.push(s);

    while (!q.empty()){
        int curr_node = q.front();
        q.pop();

        for (auto el : adj[curr_node]){
            int nod = el.first; 
            int dist_n = el.second;
            if (!viz[nod]){
                q.push(nod);
                dist[nod] = dist[curr_node] + dist_n;
                if (nod == y){
                    return dist[nod];
                }
                viz[nod] = 1;
            }
            
        }
    }

    return -1;
}



int main(){

    file_in >> n >> m >> x >> y;
    dist.resize(n+1, 0);
    adj.resize(n+1);
    viz.resize(n+1);


    for (int i = 1; i<=m; i++){
        int n1_t, n2_t, d_t;
        file_in >> n1_t >> n2_t >> d_t;
        adj[n1_t].push_back({n2_t, d_t});
        adj[n2_t].push_back({n1_t, -d_t});
    }

    dist[x] = 0;
    int rez = abs(BFS(x));
    file_out << rez;
    return 0;
}