#include<fstream.h>

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

long n,m,x,y;
long c[30001];
long long dist[30001];

struct muchie
  {long x,y;
   long d;
  };
muchie l[100101];


void parcurgere()
{long i,p,u;
long aux;
c[1]=x;
dist[x]=0;
p=u=1;
while(p<=u)
  {aux=c[p];
   p++;
   for(i=1;i<=m;i++)
     if(l[i].x==aux )
         {if( dist[l[i].y]==0)
                {u++;
                 c[p]=l[i].y;
                 dist[l[i].y]=dist[aux]+l[i].d;
                }
         }
       else if(l[i].y==aux)
               if(dist[l[i].x]==0)
                {u++;
                 c[p]=l[i].x;
                 dist[l[i].x]=dist[aux]-l[i].d;
                }
  }
fout<<dist[y];
}

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