#include <fstream>
using namespace std;
ifstream fin ("sate.in");
ofstream fout ("sate.out");
struct nod
{
    int val,dist;
    nod *urm;
} *p,*l[1000000];
int n,m,x,y,v[1000000],q[1000000];
void bf (int t)
{
    int x,prim,ultim;
    nod *p;
    prim=ultim=1;
    q[1]=t;
    while (prim<=ultim)
    {
        x=q[prim];
        for(p=l[x];p;p=p->urm)
        {
            if(v[p->val]==0)
            {
                v[p->val]=p->dist+v[x];
                q[++ultim]=p->val;
            }
        }
        prim++;
    }
}
int main()
{
    int i,j,k,c;
    fin>>n>>m>>x>>y;
    for (k=1;k<=m;k++)
    {
        fin>>i>>j>>c;
        p=new nod;
        p->val=j;
        p->dist=c;
        p->urm=l[i];
        l[i]=p;
        p=new nod;
        p->val=i;
        p->dist=c*(-1);
        p->urm=l[j];
        l[j]=p;
    }
    bf (x);
    fout<<v[y];
    return 0;
}
/*void bf(int s)
{
    int x; nod *p;
    pq=nq=1;
    q[pq]=s;
    while (pq<=nq)
    {
        x=q[pq];
        for(p=l[x];p;p=p->next)
        {
            if(viz[p->val]==0)
            {
                viz[p->val]=p->dist+viz[x];
                q[++nq]=p->val;
            }
        }
        pq++;
    }
}*/