#include <bits/stdc++.h>

using namespace std;

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

///           xi,  d
vector < pair <int, int> > h[30001];
queue <int> q;
int d[30001], X, Y;
/// d[i] = distanta de la X la i

int main()
{
    int i, j, n, m, x, y, dist;
    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())
    {
        x = q.front();
        q.pop();
        for (auto w : h[x])
        {
            j = w.first;
            dist = w.second;
            if (d[j] == -1)
            {
                d[j] = d[x] + dist;
                q.push(j);
                if (d[Y] >= 0)
                {
                    fout << d[Y];
                    return 0;
                }
            }
        }
    }

    return 0;
}