#include <bits/stdc++.h>

using namespace std;
ifstream fin("sate.in");
ofstream fout("sate.out");
struct muchie{
    int d,u;
    muchie(int _d, int _u){
        d = _d;
        u = _u;
    }
};
bool rez;
int x,y;
vector <muchie> G[30002];
bool viz[30002];
void dfs(int v, int d){
    viz[1] = 1;
    if(rez) return;
    if(v == y){
        rez = 1;
        fout << d;
    }
    for(int i = 0; i < G[v].size(); i++){
        muchie x = G[v][i];
        if(!viz[x.u]){
            viz[x.u] = 1;
            dfs(x.u,d + x.d);
        }
    }
}
int main()
{
    int n,m,u,v,d,i;
    fin >> n >> m >> x >> y;
    for(i = 1; i <= m; i++){
        fin >> u >> v >> d;
        G[u].push_back(muchie(d,v));
        G[v].push_back(muchie(-d,u));
    }
    dfs(x,0);
    return 0;
}