#include <bits/stdc++.h>

using namespace std;
ifstream fin("sate.in");
ofstream fout("sate.out");
vector <pair<int,int>>v[30001];
bool ver[30001];
int d[30001];
queue <int>Q;
int x,y;
void bfs(int i)
{
    ver[i]=true;
    for(auto y:v[i])
    {
        if(ver[y.second]==false)
        {
            Q.push(y.second);
            d[y.second]=d[i]+y.first;
        }
    }
    Q.pop();
}
int main()
{
  int i,n,m,x1,x2,c;
  fin>>n>>m>>x>>y;
  for(i=1;i<=m;i++)
  {
      fin>>x1>>x2>>c;
      v[x1].push_back(make_pair(c,x2));
      v[x2].push_back(make_pair(-c,x1));
  }
  if(x>y)swap(x,y);
  Q.push(x);
  while(d[y]==0)
  {
      bfs(Q.front());
  }
  fout<<d[y];
}