#include <bits/stdc++.h>
#define pb push_back
using namespace std;
ifstream f ( "sate.in" );
ofstream g ( "sate.out");

vector < pair <int ,int > > v[30005];
bool viz[30005];
int drum[30005];
int n,m,x,y,ans;

void parcg (int x)
{
    queue < int > q;
    q.push(x);
    viz[x]=1;
    drum[x]=0;
    while(!q.empty())
    {
        int nod=q.front();
        q.pop();
        for(auto it:v[nod])
        {
            if(!viz[it.first])
            {
                viz[it.first]=1;
                if(it.first<nod) drum[it.first]=drum[nod]-it.second;
                else drum[it.first]=drum[nod]+it.second;
                q.push(it.first);
            }
        }
    }
}

main()
{
    f>>n>>m>>x>>y;

    for(int i=1;i<=m;i++)
    {
        int a,b,x;
        f>>a>>b>>x;
        v[a].pb({b,x});
        v[b].pb({a,x});
    }
    parcg(x);
    g<<drum[y];
    return 0;
}