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

using namespace std;

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

#define lim 30010
int n,m,x,y,a,b,c;
struct loc{int sat,d;} aux;
bool viz[lim];
vector <loc> G[lim];
int dist[lim];


void bfs(int nod)
{
    vector <loc> :: iterator it;
    for (it=G[nod].begin() ; it!=G[nod].end() ; ++it)
    {
        aux=*it;
        if(!viz[aux.sat])
        {
            viz[aux.sat]=true;
            if(aux.sat>nod)
            {
                dist[aux.sat]=dist[nod]+aux.d;
                bfs(aux.sat);
            }
            if(aux.sat<nod)
            {
                dist[aux.sat]=dist[nod]-aux.d;
                bfs(aux.sat);
            }
        }
    }
}


int main()
{
    fin>>n>>m>>x>>y;

    for(int i=1; i<=m; i++)
    {
        fin>>a>>b>>c;
        G[a].push_back({b,c});
        G[b].push_back({a,c});
    }

    dist[x]=0;
    bfs(x);
    fout<<dist[y];

    fin.close();
    fout.close();
    return 0;
}