#include<fstream.h>

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

int n,m,x,y,c[30004];
long d[30004];
int xx[100025],yy[100025];
long dist[100025];


void parcurgere()
{int p,u,aux,i;
c[1]=x;
d[x]=0;
p=u=1;
while(p<=u)
  {aux=c[p];
   p++;
   for(i=1;i<=m;i++)
     if(xx[i]==aux && d[yy[i]]==0)
      {u++;
       c[u]=yy[i];
       if(yy[i]>=aux) d[yy[i]]=d[aux]+dist[i];
          else d[yy[i]]=d[aux]-dist[i];
      }
     else
           if(yy[i]==aux && d[xx[i]]==0)
           {u++;
           c[u]=xx[i];
          if(xx[i]>=aux) d[xx[i]]=d[aux]+dist[i];
             else d[xx[i]]=d[aux]-dist[i];
          }
  }
fout<<d[y];


}

int main()
{fin>>n>>m>>x>>y;
for(int i=1;i<=m;i++)
  fin>>xx[i]>>yy[i]>>dist[i];
parcurgere();
fin.close();
fout.close();
return 0;
}