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

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

int n, m;
int p, q;
vector<pair<int, int>> adj[30004];
bitset<30004> viz;
long long distanta[30004];
void dfs(int v)
{
    viz[v] = true;
    for (auto u : adj[v])
    {
        if (!viz[u.first])
        {
            distanta[u.first] = distanta[v] + u.second;
            dfs(u.first);
        }
    }
}

int main()
{
    fin >> n >> m >> p >> q;
    for (int i = 1; i <= m; i++)
    {
        int a, b, d;
        fin >> a >> b >> d;
        adj[a].push_back({b, d});
        adj[b].push_back({a, -d});
    }
    distanta[p] = 0;
    dfs(p);
    fout << distanta[q];
    return 0;
}