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

using namespace std;

typedef pair<short,int> paer;

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

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

    short n, x, y, 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));
    }

    list<short> bf;
    short sat = x;
    bf.push_back(sat);

    while (!bf.empty()) {
        sat = bf.front();
        d = distante[sat];
        bf.pop_front();

        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] = d - p.second;
                }
                else {
                    distante[p.first] = d + p.second;
                }

                bf.push_back(p.first);
            }
        }
    }

    out<<distante[y];

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