#include <iostream>
#include <fstream>
#include <list>
#include <unordered_map>

using namespace std;

typedef pair<short,int> paer;
short x, y;
int sol;

unordered_map<short,list<paer>> vecini;
unordered_map<short,int> distante;

void df(short sat) {
    if (sat == y) {
        sol = distante[sat];
    }

    list<paer>::iterator it;

    for (it = vecini[sat].begin(); it != vecini[sat].end(); it++) {
        paer p = *it;
        if (!distante[p.first]) {
            if (sat > p.first) {
                distante[p.first] = distante[sat] - p.second;
            }
            else {
                distante[p.first] = distante[sat] + p.second;
            }

            if (p.first != x) {
                df(p.first);
            }
        }
    }
}

int main()
{
    ifstream in("sate.in");
    ofstream out("sate.out");

    short n, a, b;
    int d, i, m;
    in>>n>>m>>x>>y;

    for (i=0; i<m; i++) {
        in>>a>>b>>d;
        vecini[a].push_back(paer(b,d));
        vecini[b].push_back(paer(a,d));
    }

    df(x);
    out<<sol;

    in.close();
    out.close();
    return 0;
}