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

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

struct Edge {
    int node;
    int dist;
};

std::vector<std::vector<Edge>> graph;
std::vector<int> dist;

void bfs(int start) {
    std::queue<int> q;
    q.push(start);
    dist[start] = 1;

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

        for (auto &edge : graph[curr]) {
            int neigh = edge.node;
            int curr_to_neigh_dist = edge.dist;

            if (dist[neigh] == 0) {
                dist[neigh] = dist[curr] + curr_to_neigh_dist;
                q.push(neigh);
            }
        }
    }
}

int main() {
    int n, m, x, y;
    fin >> n >> m >> x >> y;

    graph.resize(n + 1);
    dist.resize(n + 1);

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

    bfs(x);
    fout << dist[y] - 1;

    return 0;
}