#include <fstream>
#include <queue>
#include <iostream>
#include <vector>
using namespace std;
ifstream fin ("sate.in");
ofstream fout ("sate.out");
vector <pair <int, int> > h[30002];
queue <int> q;
int d[30001], X, Y;
/// d[i] = dist de la X la i
int main()
{
    int n, m, i, x, y, dist, p;
    fin >> n >> m >> X >> Y;
    for (i = 1; i <= m; i++)
    {
        fin >> x >> y >> dist;
        h[x].push_back({y, dist});
        h[y].push_back({x, -dist});
    }
    for (i = 1; i <= n; i++)
        d[i] = -1;
    d[X] = 0;
    q.push(X);
    while (!q.empty())
    {
        p = q.front();
        q.pop();
        for (i = 0; i < h[p].size(); i++)
        {
            d[h[p][i].first] = d[p] + h[p][i].second;
            q.push(h[p][i].first);
        }
        if (d[Y] >= 0)
        {
            fout << d[Y] << "\n";
            fout.close();
            return 0;
        }
    }
    return 0;
}