#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] = dist de la X la i


int main()
{
    int i, m, x, y, dist, n, k, j;
    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;
}