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

ifstream cin ("sate.in");
ofstream cout ("sate.out");

vector < pair <int , int> > adiacenta[30001];
int distanta[30001];

int main ()
{
    int numar_noduri , numar_muchii , inceput , sfarsit;
    cin >> numar_noduri >> numar_muchii >> inceput >> sfarsit;

    while (numar_muchii--)
    {
        int nod_1 , nod_2 , distanta;
        cin >> nod_1 >> nod_2 >> distanta;
        adiacenta[nod_1].push_back({nod_2 , distanta});
        adiacenta[nod_2].push_back({nod_1 , distanta});
    }

    queue <int> candidati;
    for (candidati.push(inceput) , distanta[inceput] = 1 ; !distanta[sfarsit] ; candidati.pop())
    {
        inceput = candidati.front();
        for (auto nod_vecin : adiacenta[inceput]) {
            if (!distanta[nod_vecin.first]) {
                if (nod_vecin.first < inceput) { distanta[nod_vecin.first] = distanta[inceput] - nod_vecin.second; }
                else { distanta[nod_vecin.first] = distanta[inceput] + nod_vecin.second; }
                candidati.push(nod_vecin.first);
            }
        }
    }

    cout << distanta[sfarsit] - 1;
    cout.close(); cin.close();
    return 0;
}