#include <iostream>
#include <fstream>
#include <vector>
#include <queue>
#include <utility>
using namespace std;
ifstream f("sate.in");
ofstream g("sate.out");
vector<vector<pair<int,int>>> adj;
vector<int> dist;
int n,m,x,y;
void BFS(int source)
{
    queue<int> q;
    dist[source]=0;
    q.push(source);
    while(!q.empty())
    {
        int nod=q.front();
        q.pop();
        for(auto& pereche:adj[nod])
        {
            int vecin=pereche.first;
            int cost=pereche.second;
            if(dist[vecin]==-1)
            {
                if(vecin>nod)
                {
                    dist[vecin]=dist[nod]+cost;
                    q.push(vecin);
                }
                else
                {
                    dist[vecin]=dist[nod]-cost;
                    q.push(vecin);
                }
            }
        }

    }
}
int main()
{
    f>>n>>m>>x>>y;
    adj.resize(n+1);
    dist.resize(n+1,-1);
    for(int i=1; i<=m; i++)
    {
        int a,b,c;
        f>>a>>b>>c;
        adj[a].push_back({b,c});
        adj[b].push_back({a,c});

    }
    BFS(x);
    g<<dist[y];
    return 0;
}