#include <bits/stdc++.h>
#define Dmax 30005
using namespace std;

ifstream f("sate.in");
ofstream g("sate.out");
vector < pair < int,int > >G[Dmax];
pair < int, int > tati[Dmax];
int n,m,start,stop;
bool viz[Dmax];
queue < int > Q;

void BFS(int start)
{
    Q.push(start);
    tati[start].first = 0;
    tati[start].second = 0;
    viz[start] = true;
    while(!Q.empty())
    {
        int nod = Q.front();
        Q.pop();
        for(unsigned int i = 0; i < G[nod].size(); i++)
        {
            int Vecin = G[nod][i].first;

            if(!viz[Vecin])
            {

                viz[Vecin] = true;
                tati[Vecin].first = nod;
                tati[Vecin].second = G[nod][i].second;
                Q.push(Vecin);
            }
        }
    }


}


int main()
{
    f>>n>>m>>start>>stop;
    for(int i = 1; i <= m; i++)
    {
        int x,y,z;
        f>>x>>y>>z;
        G[x].push_back({y,z});
        G[y].push_back({x,z});
    }

    BFS(start);

int y = stop;
int sum = 0;
while(y!=start)
{
    if(y > tati[y].first)
    sum += tati[y].second;
    else
        sum -= tati[y].second;


    y = tati[y].first;
}

g<<sum;
    return 0;
}