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

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

int main()
{
    int n, m, x, y;
    fin >> n >> m >> x >> y;

    vector<vector<int> > vec(n + 1), cst(n + 1);
    for (int k = 0; k < m; k++)
    {
        int a, b, d;
        fin >> a >> b >> d;
        vec[a].push_back(b);
        cst[a].push_back(d);
        vec[b].push_back(a);
        cst[b].push_back(-d);
    }

    vector<int> dist(n + 1, 0);
    vector<bool> viz(n + 1, false);
    queue<int> q;
    q.push(x);
    viz[x] = true;

    while (!q.empty())
    {
        int nod = q.front();
        q.pop();
        for (int i = 0; i < (int)vec[nod].size(); i++)
        {
            int urm = vec[nod][i];
            if (!viz[urm])
            {
                viz[urm] = true;
                dist[urm] = dist[nod] + cst[nod][i];
                q.push(urm);
            }
        }
    }

    int rez = dist[y];
    if (rez < 0)
        rez = -rez;
    fout << rez << "\n";
    return 0;
}