#include<cstdio>
#define N 30001
typedef struct nod
{int info,cost;
struct nod *next;}Nod;
bool c[N];
Nod *l[N],*t;
int i,n,m,x,y,j,p,u,k,s[N];

void A(int i,int j,int k)
{Nod *p=new Nod;
p->info=j,p->cost=k,p->next=l[i],l[i]=p;}

int B(int i,int j)
{for(Nod *p=l[i];p;p=p->next)
if(p->info==j)
      return p->cost;
return 0;}

int main()
{FILE *f=fopen("sate.in","r"),*g=fopen("sate.out","w");
fscanf(f,"%d%d%d%d",&n,&m,&x,&y),c[x]=1;
while(m--)
      fscanf(f,"%d%d%d",&i,&j,&k),A(i,j,k),A(j,i,-k);
s[u++]=l[x]->info;
for(;p<u;p++)
      {if(c[y])
             break;
      j=s[p],c[j]=1;
      for(t=l[j];t;t=t->next)
      if(!c[t->info])       
             {s[u++]=t->info;
             if(B(j,t->info))
                    A(x,t->info,B(j,t->info)+B(x,j));
             else
                    A(x,t->info,B(x,j)-B(t->info,j));}}
fprintf(g,"%d",B(x,y));
return 0;}