#include <iostream>
#include <fstream>
#include <vector>

using namespace std;

ifstream fin ("sate.in");
ofstream fout ("sate.out");

const int MAX = 100024;
int dist[MAX];

struct sat
{
    int nod, cost;
};

vector <sat> v[MAX];

int n, m, s, f, x, y, c;

void dfs(int node)
{
    int marime = v[node].size();
    for(int i=0; i<marime; i++)
        if(dist[v[node][i].nod]==0)
        {
            if(v[node][i].nod < node)
                dist[v[node][i].nod] = dist[node] - v[node][i].cost;
            else
                dist[v[node][i].nod] = dist[node] + v[node][i].cost;
            dfs(v[node][i].nod);
        }
}
int main()
{
    fin>>n>>m>>s>>f;
    for(int i=1; i<=m; i++)
    {
        fin>>x>>y>>c;
        v[x].push_back({y, c});
        v[y].push_back({x, c});
    }
    dist[s]=1;
    dfs(s);
    fout<<dist[f]-1;
    return 0;
}