#include <bits/stdc++.h>

using namespace std;

#define pb push_back

int n, m, x, y;
vector <int> dist;
vector <vector <pair <int, int> > > graph;

void dfs(int node, int parent, int cost)
{
    dist[node] = dist[parent] + cost;
    for(auto neighbour : graph[node])
        if(!dist[neighbour.first])
        {
            dfs(neighbour.first, node, neighbour.second);
        }
}

int main()
{
    ios_base :: sync_with_stdio(0);
    cin.tie(0);

    freopen("sate.in", "r", stdin);
    freopen("sate.out", "w", stdout);

    cin >> n >> m >> x >> y;
    graph.resize(n + 1);
    dist.resize(n + 1, 0);

    for(int i = 1; i <= m; i ++)
    {
        int a, b, cost;
        cin >> a >> b >> cost;
        graph[a].pb({b, cost});
        graph[b].pb({a, -cost});
    }

    dist[x] = 0;
    dfs(x, x, 1);
    cout << dist[y] - 1;
    return 0;
}