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

int main() {
    std::ifstream input("sate.in");
    std::ofstream output("sate.out");

    int n, m, x, y;

    input >> n >> m >> x >> y;

    std::vector<std::vector<std::pair<int, int>>> graph;

    graph.resize(n + 1);

    for (int i = 1; i <= m; ++i) {
        int p, q, d;
        input >> p >> q >> d;

        graph[p].push_back({q, d});
        graph[q].push_back({p, d});
    }

    std::vector<int> dist;
    dist.resize(n + 1, 0);

    dist[x] = 0;

    std::vector<bool> visited;
    visited.resize(n + 1, false);

    visited[x] = true;
    std::queue<int> queue;
    queue.push(x);

    while (!queue.empty()) {
        int top = queue.front();
        queue.pop();

        for (auto const &node: graph[top]) {
            if (!visited[node.first]) {
                queue.push(node.first);
                visited[node.first] = true;
                dist[node.first] = dist[top] + (node.first < top ? -1 : 1) * node.second;
            }
        }
    }
    output << dist[y];

    return 0;
}