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

const int NMAX = 100024;

int n, m;
vector<pair<int, int>> adj[NMAX];
int dist[NMAX];
bool viz[NMAX];

int bfs(int start, int target)
{
    queue<int> q;
    q.push(start);
    viz[start] = true;
    dist[start] = 0;

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

        for (auto vecin : adj[nod])
        {
            int next = vecin.first;
            int d = vecin.second;

            if (!viz[next])
            {
                viz[next] = true;

                if (nod < next)
                    dist[next] = dist[nod] + d;
                else
                    dist[next] = dist[nod] - d;

                if (next == target)
                    return dist[next];

                q.push(next);
            }
        }
    }

    return -1;
}

int main()
{
    freopen("sate.in", "r", stdin);
    freopen("sate.out", "w", stdout);

    int x, y;
    scanf("%d %d %d %d", &n, &m, &x, &y);

    for (int i = 0; i < m; i++)
    {
        int a, b, d;
        scanf("%d %d %d", &a, &b, &d);

        adj[a].push_back({b, d});
        adj[b].push_back({a, d});
    }

    printf("%d\n", bfs(x, y));
    return 0;
}