#include <fstream>
#include <vector>
#include <queue>
#include <utility>

using namespace std;

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

const int NMax = 30000;

vector < pair<int, int> > G[NMax + 5]; queue <int> Q;
int D[NMax + 5], N, M, X, Y;

void BFS(int Nod)
{
    Q.push(Nod), D[Nod] = 0;

    while(!Q.empty()) {
         Nod = Q.front(); Q.pop();

         for(auto Vecin : G[Nod])
         {
             if(D[Vecin.first] == -1)
                D[Vecin.first] = D[Nod] + Vecin.second, Q.push(Vecin.first);
         }
    }
}

int main()
{
    fin >> N >> M >> X >> Y;

    if(X > Y) swap(X, Y);

    for(int i = 1, a, b, c; i <= M; i++)
    {
        fin >> a >> b >> c;

        G[a].push_back({b,  c});
        G[b].push_back({a, -c});
    }
    for(int i = 1; i <= N; i++)
        D[i] = -1;

    BFS(X); fout << D[Y] << '\n';

    fin.close();
    fout.close();

    return 0;
}