#include <vector>
#include <queue>
#include <fstream>
#include <utility>
#include <iostream>
#define NMAX 30004
using namespace std;

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

vector<pair<int, int> >v[NMAX];
queue<int>coada;
int p[NMAX];
int n, m, x, y;

int main()
{

    fin >> n >> m >> x >> y;
    int a, b, d;
    for (int i = 1; i <= m; ++i)
    {
        fin >> a >> b >> d;
        v[a].push_back(make_pair(b, d));
        v[b].push_back(make_pair(a, d));
    }
    coada.push(x);

    int nod, rs = 0;
    while (!coada.empty())
    {
        nod = coada.front();
        //   cout << nod << "\n";
        for (int i = 0; i < v[nod].size(); ++i)
        {
            if (!p[v[nod][i].first])
            {
                if (v[nod][i].first > nod)
                {
                    p[v[nod][i].first] = p[nod] + v[nod][i].second;
                }
                else
                {
                    p[v[nod][i].first] = p[nod] - v[nod][i].second;
                }
                coada.push(v[nod][i].first);

            }
        }
        coada.pop();
    }
    fout << p[y];
    return 0;
}