#include <iostream>
#include <vector>
#include <queue>
#include <fstream>

using namespace std;

int vizitat[100000], suma[10000];

int main()
{
    ifstream f("sate.in");
    ofstream g("sate.out");

    vector<int> v[10000], dist[10000];
    queue<int> q;

    int n, m, i, j, start, sfarsit, x, y, nod, sursa, col = 1, d, k;

    f >> n >> m >> start >> sfarsit;

    for (i = 0; i < m; i++)
        {
            f >> x >> y >> d;

            v[x].push_back(y);
            dist[x].push_back(d);

            v[y].push_back(x);
            dist[y].push_back(d);
        }

    sursa = start;

    if(sursa > n)
        return 0;

    vizitat[sursa] = col;
    q.push(sursa);
    d = 0;

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

         col = vizitat[nod];

         if(col == 1)
            col = 2;
         else
            col = 1;

         for(i = 0; i < v[nod].size(); i++)
            if(vizitat[v[nod][i]] == 0)
                {
                    vizitat[v[nod][i]] = col;

                    if(nod > v[nod][i])
                        suma[v[nod][i]] = suma[nod] - dist[nod][i];

                    if(nod < v[nod][i])
                        suma[v[nod][i]] = suma[nod] + dist[nod][i];

                    q.push(v[nod][i]);
                }
        }

    g << suma[sfarsit];

    return 0;
}