#include <fstream>
#include <vector>
#include <queue>

using namespace std;


const int NMAX = 30003;

int n,m,x,y;
vector<pair<int,int>> muchii[NMAX + 1];
vector<int> dist(NMAX + 1);

void bf(int nod) {
    int nodcrt;
    int viz[NMAX + 1];
    queue<int> q;

    viz[nod] = 1;
    q.push(nod);

    while (!q.empty()) {
        nodcrt = q.front();
        q.pop();

        for (auto &elem : muchii[nodcrt]) {
            int vecin = elem.first;
            int d = elem.second;

            if (!viz[vecin]) {
                viz[vecin] = 1;
                q.push(vecin);
                dist[vecin] = dist[nodcrt] + d;
            }
        }
    }
}

int main() {

    ifstream fin("sate.in");
    ofstream fout("sate.out");

    fin >> n >> m >> x >> y;

    for (int i = 0; i < m; i++) {
        int a,b,d;
        fin >> a >> b >> d;
        muchii[a].emplace_back(b,d);
        muchii[b].emplace_back(a,-d);
    }

    bf(x);

    fout << dist[y];

    return 0;
}