#include <bits/stdc++.h>
using namespace std;

/**
{1}
*/
vector< pair<int, int> > h[30001];
int viz[30001], n, m, X, Y;
ifstream fin("sate.in");
ofstream fout("sate.out");

int BFS(int k)
{
    queue< pair<int, int> > q;
    int dist;
    q.push({ k, 0 });
    viz[k] = 1;
    while (!q.empty())
    {
        k = q.front().first;
        dist = q.front().second;
        q.pop();
        for (auto e : h[k])
            if (viz[e.first] == 0)
            {
                viz[e.first] = 1;
                q.push({ e.first, dist + e.second });
                if (e.first == Y)
                    return dist + e.second;
            }
    }
    return 0;
}

int main()
{
    int i, j, k, dist, m;
    fin >> n >> m >> X >> Y;
    for (k = 1; k <= m; k++)
    {
        fin >> i >> j >> dist;
        if (i > j) swap(i, j);
        h[i].push_back({ j, dist });
        h[j].push_back({ i, -dist });
    }
    fout << BFS(X) << "\n";
    fout.close();
    return 0;
}