#include <fstream>
#include <vector>
#include <queue>
using namespace std;
ifstream fin("sate.in");
ofstream fout("sate.out");
int n,m,i,j,k,x,c,p1,p2,d[30005],aux,v[30005];
struct muchie{int nod,val;}p;
vector<muchie>V[30005];
queue<int>C;
int main()
{fin>>n>>m>>p1>>p2;
 for(i=1;i<=m;i++)
    {fin>>x>>p.nod>>p.val;
     V[x].push_back(p);
     aux=x;
     x=p.nod;
     p.nod=aux;
     V[x].push_back(p);
    }
 C.push(p1);
 while(C.size()>0&&C.front()!=p2)
      {c=C.front();
       C.pop();
       v[c]=1;
       for(i=0;i<V[c].size();i++)
          {if(v[V[c][i].nod]==0)
             {C.push(V[c][i].nod);
              v[V[c][i].nod]=1;
              if(V[c][i].nod>c)d[V[c][i].nod]=d[c]+V[c][i].val;
              else d[V[c][i].nod]=d[c]-V[c][i].val;
             }
          }
      }
 if(d[p2]<0)d[p2]=d[p2]*(-1);
 fout<<d[p2];
}