#include <bits/stdc++.h>
using namespace std;

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

vector < pair < int, int > > h[30001];
queue < int > Q;
int d[30001], X, Y, n, m;

int main()
{
    int i, m, x, y, dist, 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] << "\n";
                return 0;
            }
            }
        }
    }

    return 0;
}