#include <bits/stdc++.h>

using namespace std;

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

const int N = 30005;

vector <pair <int, int>> g[N];

int n, m, x, y;

int dist[N];

int main()
{
    in >> n >> m >> x >> y;
    for(int i = 1; i <= m; i++)
    {
        int unu, doi, w;
        in >> unu >> doi >> w;
        g[unu].push_back({doi, w});
        g[doi].push_back({unu, -w});
    }
    queue <pair <int, int>> q;
    q.push({x, 0});
    for(int i = 1; i <= n; i++)
        dist[i] = -1;
    dist[x] = 0;
    while(q.size())
    {
        pair <int, int> curr = q.front();
        q.pop();
        for(auto it:g[curr.first])
        {
            if(dist[it.first] == -1)
            {
                dist[it.first] = it.second + dist[curr.first];
                q.push(it);
            }
        }
    }
    out << dist[y];
    return 0;
}