#include <cstdio>
#include <vector>
#include <queue>

using namespace std;
FILE *fin, *fout;

struct edge
{
    int dest, cost;
};

#define NMAX 30000
int d[NMAX + 5];

queue <int> q;

vector <edge> graph[NMAX + 5];

void bfs(int nod)
{
    d[nod] = 0;
    q.push(nod);

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

        for(int i = 0; i < graph[nod].size(); i++)
        {
            int neigh = graph[nod][i].dest;
            int cost = graph[nod][i].cost;

            if(d[neigh] == -1 or d[nod] + cost < d[neigh])
            {
                d[neigh] = d[nod] + cost;
                q.push(neigh);
            }
        }
    }
}

int main()
{
    fin = fopen("sate.in", "r");
    fout = fopen("sate.out", "w");

    int n, m, x, y;
    fscanf(fin, "%d%d%d%d", &n, &m, &x, &y);

    int u, v, cost, i;
    edge a_node;

    for(i = 0; i < m; i++)
    {
        fscanf(fin, "%d%d%d", &u, &v, &cost);
        a_node.dest = v;
        a_node.cost = cost;
        graph[u].push_back(a_node);
        a_node.dest = u;
        a_node.cost = -cost;
        graph[v].push_back(a_node);
    }

    /*for(int i = 1; i <= n; i++)
    {
        fprintf(fout , "%d: " , i);
        for(auto it : graph[i])
            fprintf(fout, "%d %d ", it.dest , it.cost);
        fprintf(fout, "\n");
    }*/

    for(i = 1; i <= n; i++)
        if(d[i] == 0)
            d[i] = -1;

    bfs(x);

    //for(i = 1; i <= n; i++)
       // fprintf(fout, "%d ", d[i]);

    fprintf(fout, "%d", d[y]);

    fclose(fin);
    fclose(fout);
    return 0;
}