#include <iostream>
#include <fstream>
#include <map>
#include <queue>
#include <vector>
#include <string>
#include <cmath>
#include <cstring>
#include <set>
using namespace std;
ifstream f("sate.in");
ofstream g("sate.out");
///#define f cin
///#define g cout
#define NMAX 110000
int n, m, x, y;
vector<pair<int,int>> adj[NMAX];
int dist[NMAX];
int viz[NMAX];
queue<int> q;
void bfs()
{
    while (!q.empty())
    {
        int curent = q.front();
        q.pop();
        viz[curent] = 1;
        for (auto vecin : adj[curent])
        {
            if (!viz[vecin.first])
            {
                viz[vecin.first] = 1;
                dist[vecin.first] = dist[curent] + vecin.second;
                q.push(vecin.first);
            }
        }
    }
}
int main()
{
    f >> n >> m >> x >> y;
    for (int i = 1; i <= m; i++)
    {
        int a, b, w;
        f >> a >> b >> w;
        adj[a].push_back({ b,w });
        adj[b].push_back({ a,-w });
    }
    q.push(1);
    if (adj[1].size() == 0)
        return 0;
    q.push(x);
    bfs();
    g << dist[y];
    return 0;
}