#include <iostream>
#include <fstream>
#include <deque>
#include <unordered_map>

using namespace std;

typedef pair<short,int> paer;

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

    unordered_map<short,deque<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));
    }

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

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

        for (i=0; i<vecini[sat].size(); i++) {
            if (!distante[vecini[sat][i].first]) {
                if (sat > vecini[sat][i].first) {
                    distante[vecini[sat][i].first] = d - vecini[sat][i].second;
                }
                else {
                    distante[vecini[sat][i].first] = d + vecini[sat][i].second;
                }

                bf.push_back(vecini[sat][i].first);
            }
        }
    }

    out<<distante[y];

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