#pragma GCC optimize("O3")

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

using namespace std;

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

int n, m, x, y;
vector<pair<int, int>> G[30005];

bool visited[30005];
queue<pair<int, int>> q;

int main() {
	ios_base::sync_with_stdio(false);
	in.tie(0);
	out.tie(0);
	in >> n >> m >> x >> y;
	for (int i = 1; i <= m; i++) {
		int a, b, d;
		in >> a >> b >> d;
		G[a].push_back({ b, d });
		G[b].push_back({ a, -d });
	}
	q.push({ x, 0 });
	visited[x] = 1;
	while (!q.empty()) {
		pair<int, int> current = q.front();
		if (current.first == y) {
			out << current.second;
			return 0;
		}
		q.pop();
		for (auto edge : G[current.first]) {
			pair<int, int> newState = { edge.first, current.second + edge.second };
			if (!visited[newState.first]) {
				visited[newState.first] = 1;
				q.push(newState);
			}
		}
	}
}