#include <bits/stdc++.h>

using namespace std;
ifstream fin("sate.in");
ofstream fout("sate.out");
int N,M,X,Y;
vector<int>ad[30001],cost[30001];
int dist[30001];
bool viz[30001];
void read ()
{int i,x,y,c;
 fin>>N>>M>>X>>Y;
 for(i=1;i<=M;i++)
    {fin>>x>>y>>c;
     ad[x].push_back(y);
     cost[x].push_back(c);
     ad[y].push_back(x);
     cost[y].push_back(c);
    }
 }

inline void dfs(int nod)
{viz[nod]=1;
 int w;
 for(int i=0;i<ad[nod].size();i++)
   if(viz[ad[nod][i]]==0)
    {w=ad[nod][i];
     if(w<nod)dist[w]=dist[nod]-cost[nod][i];
     else dist[w]=dist[nod]+cost[nod][i];
     dfs(w);
    }
}
int main()
{
    read();
    dfs(X);
    fout<<dist[Y];
    return 0;
}