#include <bits/stdc++.h>
using namespace std;
ifstream fin ("sate.in");
ofstream fout ("sate.out");

int n, m;
vector <pair <int, int>> g[100001];
int dist[100001];

void dfs(int sursa)
{
    for (auto vecin : g[sursa])
        if (!dist[vecin.first])
        {
            if (vecin.first < sursa)
                dist[vecin.first] = dist[sursa] - vecin.second;
            else
                dist[vecin.first] = dist[sursa] + vecin.second;
            dfs(vecin.first);
        }
}

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

    for (int i = 1; i <= m; i++)
    {
        int a, b, cost;
        fin >> a >> b >> cost;
        g[a].push_back({b, cost});
        g[b].push_back({a, cost});
    }
    dist[x] = 0;
    dfs(x);
    fout << dist[y];
    return 0;
}