#include <iostream>
#include <vector>
#include <fstream>
#include <queue>
using namespace std;
#define MAX 30005
int abs(int n)
{
    if(n<0)
         return -n;
    return n;
}
ifstream fin("sate.in");
ofstream fout("sate.out");
vector < pair < int, int > > v[MAX];
queue  < int > coada;
bool viz[MAX];
int cost[MAX];
int main()
{
    int n, m, a, b,c,x,y;
    fin >> n >> m >> x >> y;
    for (int i = 1; i<=m; i++) {
            fin >> a >> b >> c;
            v[a].push_back(make_pair(c,b));
            v[b].push_back(make_pair(c,a));
    }
    coada.push(x);
    viz[x]=true;
    while (!coada.empty()) {
        int nod = coada.front();
        coada.pop();
        for (int i = 0; i<v[nod].size(); i++) {
            int vecin = v[nod][i].second;
            if (viz[vecin]==false) {
                coada.push(vecin);
                viz[vecin]=true;
                if(vecin<nod)
                    cost[vecin]=cost[nod]-v[nod][i].first;
                else cost[vecin]=cost[nod]+v[nod][i].first;
            }
        }
    }
    fout << abs( cost[y] ) << "\n";
    fin.close();
    fout.close();
    return 0;
}