#include <fstream>
#include <queue>
#include <vector>
#include <iostream>
#include <utility>
using namespace std;

ifstream f("sate.in");
ofstream g("sate.out");

int n,m,X,Y,dist[30001];

bool viz[100001];

vector< pair<int,int> >a[30001];

void bfs(int src)
{
    viz[src]=true;
    queue< int >q;
    q.push(src);
    while(!q.empty())
    {

        int nod = q.front();
        q.pop();
        viz[nod]=true;

        for(int i=0;i<a[nod].size();++i)
            if(!viz[a[nod][i].first])
            {
                if(a[nod][i].first>nod)
                    dist[a[nod][i].first]=dist[nod]+a[nod][i].second;
                else
                    dist[a[nod][i].first]=dist[nod]-a[nod][i].second;
                q.push(a[nod][i].first);
            }



    }

}

int main()
{
    f>>n>>m>>X>>Y;
    for(int i=1;i<=m;++i)
    {

        int x,y,d;
        f>>x>>y>>d;
        a[x].push_back(make_pair(y,d));
        a[y].push_back(make_pair(x,d));
    }
    bfs(X);
    g<<dist[Y];
    return 0;
}