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

using namespace std;

typedef pair<short,int> paer;

struct nod {
    paer p;
    nod *next;
};

struct sNod {
    short sat;
    sNod *next;
};

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

    unordered_map<short,nod*> 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;

        nod *l = new nod;
        l->p = paer(b, d);
        l->next = vecini[a];
        vecini[a] = l;

        l = new nod;
        l->p = paer(a, d);
        l->next = vecini[b];
        vecini[b] = l;
    }

    sNod *bf = new sNod;
    bf->sat = x;

    while (bf) {
        short sat = bf->sat;
        d = distante[sat];
        bf = bf->next;

        nod *l = vecini[sat];
        while (l) {
            paer p = l->p;

            if (!distante[p.first]) {
                if (sat > p.first) {
                    distante[p.first] = d - p.second;
                }
                else {
                    distante[p.first] = d + p.second;
                }

                if (p.first == y) {
                    goto sol;
                }
                sNod *sn = new sNod;
                sn->sat = p.first;
                sn->next = bf;
                bf = sn;
            }

            l = l->next;
        }
    }

    sol:
    out<<distante[y];

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