#include <bits/stdc++.h>
#define cin ci
#define cout co
using namespace std;
ifstream cin("sate.in");
ofstream cout("sate.out");
int n, m, s, f, x, y, z;
vector<vector<pair<int, int>>> graf;
vector<int> dist;
queue<int> q;
void bfs(int node)
{
    dist[node] = 1;
    q.push(node);
    while(!q.empty())
    {
        node = q.front();
        q.pop();
        for(auto &[next, d]:graf[node])
            if(!dist[next])
            {
                q.push(next);
                if(node > next)
                    dist[next] = dist[node] - d;
                else
                    dist[next] = dist[node] + d;
            }
    }
}
int main()
{
    cin >> n >> m >> s >> f;
    graf.assign(n+1 ,vector<pair<int ,int>>());
    dist.resize(n+1);
    for(int i=1; i<=m; i++)
    {
        cin >> x >> y >> z;
        graf[x].push_back({y, z});
        graf[y].push_back({x, z});
    }
    bfs(s);
    cout << dist[f] - 1;
    return 0;
}