#include <fstream>
#include <vector>

using namespace std;

ifstream fin("sate.in");
ofstream fout("sate.out");

const int maxn = 30000;
vector<pair<int,int>> adj[maxn + 1];
int dist[maxn + 1];

void dfs(int nod, int d) {
    dist[nod] = d;
    for (auto child: adj[nod])
        if (dist[child.first] == -1)
            dfs(child.first, d + child.second);
}

int main() {
    int n, m, x, y;
    fin >> n >> m >> x >> y;
    for (int i = 0; i < m; i++) {
        int u, v, c;
        fin >> u >> v >> c;
        adj[u].push_back(make_pair(v, c));
        adj[v].push_back(make_pair(u, -c));
    }
    for (int i = 1; i <= n; i++)
        dist[i] = -1;
    dfs(x, 0);
    fout << dist[y] << endl;

    return 0;
}