#include <bits/stdc++.h>

using namespace std;
ifstream fin("sate.in");
ofstream fout("sate.out");
vector<pair<int,int>>l[30005];
int n,m,x,y,xs,ys,c,sum,viz[30005],d[30005];
void bfs(int nod)
{
    d[nod]=0;
    queue<int>q;
    q.push(nod);
    viz[nod]=1;
    while(!q.empty())
    {
        auto curr=q.front();
        q.pop();
        for(auto next:l[curr])
           {
             if(!viz[next.first])
             {
                 viz[next.first]=1;
                 q.push(next.first);
                 if(next.first>curr)
                     d[next.first]=next.second+d[curr];
                 else
                    d[next.first]=d[curr]-next.second;

             }
           }
    }
}
int main()
{
   fin>>n>>m>>xs>>ys;
   for(int i=1;i<=m;i++)
   {
       fin>>x>>y>>c;
       l[x].push_back({y,c});
       l[y].push_back({x,c});
   }
   bfs(xs);
   fout<<d[ys];
}