#include <fstream>
#include <vector>
#include <queue>
using namespace std;
ifstream fin("sate.in");
ofstream fout("sate.out");

int n, m;
vector<vector<pair<int, int>>> adj;
vector<int> viz;
vector<int> dist;

int BFS(int src, int fin)
{
    queue<int> q;
    q.push(src);
    dist[src] = 0;
    viz[src] = 1;
    int dist_fin = 0;

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

        for (auto &vecin : adj[nod])
        {
            int i = vecin.first;
            int cost = vecin.second;
            if (!viz[i])
            {
                viz[i] = 1;

                if (nod == src)
                {
                    dist[i] = cost;
                }
                if (i < nod)
                {
                    dist[i] = dist[nod] - cost;
                }
                if (i > nod)
                {
                    dist[i] = dist[nod] + cost;
                }
                q.push(i);
            }
        }
    }
    return dist[fin];
}

int main()
{
    int x, y;
    fin >> n >> m >> x >> y;
    adj.resize(n + 5);
    viz.resize(n + 5);
    dist.resize(n + 5);

    for (int i = 1; i <= n; i++)
    {
        dist[i] = 0;
        viz[i] = 0;
    }

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

    fout << BFS(x, y);
    return 0;
}