#include <bits/stdc++.h>

using namespace std;
ifstream fin("sate.in");
ofstream fout("sate.out");
const int nmax = 3e4+5;

vector<pair<int, int>> vecini[nmax];
queue<int> q;
int n, m, start, stop, dist[nmax];
bool viz[nmax];

int main()
{
    fin>>n>>m>>start>>stop;
    for(int i = 1; i <= m; ++i)
    {
        int x, y, d;
        fin>>x>>y>>d;
        vecini[x].push_back({y, d});
        vecini[y].push_back({x, -d});
    }

    q.push(start);
    viz[start] = true;
    while(!q.empty())
    {
        int nod = q.front();;
        if(nod == stop)
            break;
        q.pop();
        for(auto i : vecini[nod])
        {
            if(!viz[i.first])
            {
                dist[i.first] = dist[nod] + i.second;
                viz[i.first] = true;
                q.push(i.first);
            }
        }
    }
    fout<<dist[stop];

    return 0;
}