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

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

int n,m,x,y;
int d[maxn],viz[maxn];
vector <pair<int,int> > lista[maxn];
queue <int> q;

void citire()
{
    fin>>n>>m>>x>>y;
    int a,b,c;
    for(int i=1;i<=m;i++)
    {
        fin>>a>>b>>c;
        lista[a].push_back({b,c});
        lista[b].push_back({a,c});
    }
}

void bfs(int node)
{
    q.push(node);
    viz[node]=1;
    while(!q.empty())
    {
        int nod=q.front();
        q.pop();
        for(int i=0;i<lista[nod].size();i++)
        {
            if(!viz[lista[nod][i].first])
            {
                viz[lista[nod][i].first]=1;
                q.push(lista[nod][i].first);
                if(lista[nod][i].first>nod)
                    d[lista[nod][i].first]=d[nod]+lista[nod][i].second;
                else
                    d[lista[nod][i].first]=d[nod]-lista[nod][i].second;
                if(lista[nod][i].first==y)
                      fout<<d[y];
            }
        }
    }
}

int main()
{
    citire();
    bfs(x);
    return 0;
}