// O(n+m)
#include <iostream>
#include <fstream>
#include <vector>
#include <queue>
#include <cstdlib>
using namespace std;

const int NMAX = 30005;

struct Nod
{
    int vecin;
    int distanta;
};

vector<Nod> adj[NMAX];
int distanta[NMAX];
bool viz[NMAX];

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

    if (!fin || !fout) return 1;

    int n, m, X, Y;
    fin >> n >> m >> X >> Y;

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

        adj[a].push_back({b, d});
        adj[b].push_back({a, d});
    }

    queue<int> q;

    q.push(X);
    viz[X] = true;
    distanta[X] = 0;

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

        for (Nod x : adj[nod])
        {
            int vecin = x.vecin;
            int d = x.distanta;

            if (!viz[vecin])
            {
                viz[vecin] = true;

                if (vecin > nod)
                    distanta[vecin] = distanta[nod] + d;
                else
                    distanta[vecin] = distanta[nod] - d;

                q.push(vecin);
            }
        }
    }

    fout << abs(distanta[Y]);

    return 0;
}