#include <iostream>
#include <vector>
#include <list>
#include <queue>
#include <stdio.h>
using namespace std;

#define inf 21000000

struct nod_g{
    int nod, cost;
};
vector < list<nod_g> > graf;
vector <unsigned int> cost;
vector <bool> vizitat;
list <nod_g>::iterator it;

int n, m, x, y;
nod_g temp;

void citire(){
    int x1, y1, c;
    scanf("%d %d %d %d",&n,&m,&x,&y);
    graf.resize(n+1);
    cost.resize(n+1, inf);
    vizitat.resize(n+1);

    for(int i = 1; i <= m; ++i){
        scanf("%d %d %d",&x1,&y1,&c);
        temp.nod = y1; temp.cost = c;
        graf[x1].push_back(temp);
        temp.nod = x1; temp.cost = -c;
        graf[y1].push_back(temp);
    }
    fclose(stdin);
}

void bfs(){
    int first = x;
    queue <unsigned int> coada;
    coada.push(x);
    cost[x] = 0;
    vizitat[x] = 1;

    while(!coada.empty()){
        for(it = graf[first].begin(); it != graf[first].end(); ++it)
            if(!vizitat[it->nod]){
                vizitat[it->nod] = 1;
                coada.push(it->nod);
                cost[it->nod] = cost[first] + it->cost;
                if(it->nod == y){
                    printf("%u",cost[y]);
                    return;
                }
            }
        coada.pop();
        first = coada.front();
    }
}

int main(){
    freopen("sate.in","r",stdin);
    freopen("sate.out","w",stdout);
    citire();
    bfs();
    fclose(stdout);

    return 0;
}