#include <iostream>
#include<fstream>
using namespace std;
 
int n,m,x,y,i,j,k,pq,nq,d;
int viz[100000],q[100000];
 
struct nod
{
    int val,dist;
    nod *next;
};
 
nod *p,*l[1000000];
 
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)
            {
                if(p->val>x) viz[p->val]=p->dist+viz[x];
                else viz[p->val]=viz[x]-p->dist;
                q[++nq]=p->val;
            }
        }
        pq++;
    }
}
 
int main()
{
    ifstream f("sate.in");
    ofstream g("sate.out");
    f>>n>>m>>x>>y;
    for(k=1;k<=m;k++)
    {   f>>i>>j>>d;
        p=new nod;
        p->val=j;
        p->dist=d;
        p->next=l[i];
        l[i]=p;
        p=new nod;
        p->val=i;
        p->dist=d;
        p->next=l[j];
        l[j]=p;
    }
    bf(x);
    g<<viz[y];
}