#include <fstream>
#include <queue>
using namespace std;
ifstream cin("sate.in");
ofstream cout("sate.out");
int n, m, s, f, x, y, d;
vector<vector<pair<int, int>>> graf;
vector<int> viz;
void bfs()
{
    queue <int> q;
    q.push(s);
    viz[s] = 1;
    while(!q.empty())
    {
        int nod = q.front();
        q.pop();
        for(pair<int,int> i : graf[nod])
        {
            int vec = i.first;
            int cost = i.second;
            if(!viz[vec])
            {
                viz[vec] = viz[nod] + cost;
                q.push(vec);
            }
        }

    }
}
int main()
{
    cin >> n >> m >> s >> f;
    graf.assign(n+1, vector<pair<int,int>>());
    viz.resize(n+1);
    while(m--)
    {
        cin >> x >> y >> d;
        graf[x].push_back({y, d});
        graf[y].push_back({x, -d});
    }
    bfs();
    cout << viz[f]-1;
    return 0;
}