#include <iostream>
#include <fstream>
#include <vector>
#include <queue>

using namespace std;

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

int main()
{
    int n, m, x, y;
    fin >> n >> m >> x >> y;

    vector<vector<pair<int, long long>>> graf(n + 1);

    for (int i = 0; i < m; i++)
    {
        int a, b;
        long long d;

        fin >> a >> b >> d;

        graf[a].push_back({b, d});
        graf[b].push_back({a, -d});
    }

    vector<long long> dist(n + 1);
    vector<bool> vizitat(n + 1, false);

    queue<int> q;

    q.push(x);
    vizitat[x] = true;
    dist[x] = 0;

    while (!q.empty())
    {
        int nod = q.front();
        q.pop();

        if (nod == y)
            break;

        for (auto vecin : graf[nod])
        {
            int urmator = vecin.first;
            long long cost = vecin.second;

            if (!vizitat[urmator])
            {
                vizitat[urmator] = true;
                dist[urmator] = dist[nod] + cost;

                q.push(urmator);
            }
        }
    }

    fout << dist[y] << '\n';

    fin.close();
    fout.close();

    return 0;
}