#include <fstream>
using namespace std;
ifstream f("sate.in");
ofstream g("sate.out");
int st[300001], N, M, prim, ultim, x1, y1, p, i, x, y;
struct nod{
    int val,pret;
    nod *urm;
} *v[30001], *t;
struct coada {
    int n,p;
};
coada viz[30001];
void add(int x1,int y1,int p){
    nod *t;
    t=new nod;
    t->val=y1;
    t->pret=p;
    t->urm=v[x1];
    v[x1]=t;
    t=new nod;
    t->val=x1;
    t->pret=p;
    t->urm=v[y1];
    v[y1]=t;
}
void citire(int &N, int &M, int &x, int &y){
    f >> N >> M >> x >> y;
    for (i = 1;i <= M; i++){
      f >> x1 >> y1 >> p;
      add (x1,y1,p);
    }
}
int main()
{
    citire( N, M, x, y);
    viz[x].n = 1;
    prim = 1;
    ultim = 1;
    st[prim] = x;
    while(!viz[y].n){
        for (t = v[st[prim]]; t; t=t->urm)
            if (!viz[t->val].n){
                viz[t->val].n = 1;
                if (t->val > st[prim])
                    viz[t->val].p = viz[st[prim]].p + t->pret;
                else
                    viz[t->val].p = viz[st[prim]].p - t->pret;
                st[++ultim] = t->val;
                if (t->val == y)
                    break;
            }
            prim++;


    }
    g << viz[y].p;
}