#include <fstream>
#include <vector>
#include <queue>
#include <cmath>

using namespace std;

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

const int MAXN = 30005;

struct Muchie {
    int nod;
    int cost;
};

vector<Muchie> adj[MAXN];
int poz[MAXN];
bool viz[MAXN];

int main() {
    ios_base::sync_with_stdio(false);
    fin.tie(NULL);

    int n, m, x, y;
    if (!(fin >> n >> m >> x >> y)) return 0;

    for (int k = 0; k < m; ++k) {
        int i, j, d;
        fin >> i >> j >> d;
        // i < j, deci poz[j] = poz[i] + d si poz[i] = poz[j] - d
        adj[i].push_back({j, d});
        adj[j].push_back({i, -d});
    }

    queue<int> q;
    q.push(x);
    viz[x] = true;
    poz[x] = 0;

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

        if (u == y) {
            break;
        }

        for (const auto& edge : adj[u]) {
            int v = edge.nod;
            int cost = edge.cost;

            if (!viz[v]) {
                viz[v] = true;
                poz[v] = poz[u] + cost;
                q.push(v);
            }
        }
    }

    fout << abs(poz[y] - poz[x]) << "\n";

    return 0;
}