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

using namespace std;

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

struct graf {
	int nod, cost;
};

int n, m, X, Y;
vector <graf> G[30005];
bool viz[30005];
int dist[30005];

void bfs(int x)
{
	queue <int> que;
	
	dist[x] = 0;
	viz[x] = 1;
	que.push(x);

	while (!que.empty())
	{
		int vf = que.front();

		for (auto it : G[vf])
		{
			if (viz[it.nod] == 0)
			{
				viz[it.nod] = 1;
				que.push(it.nod);
				dist[it.nod] = dist[vf] + it.cost;
			}
		}

		que.pop();
	}
}

int main()
{
	fin >> n >> m >> X >> Y;
	for (int i = 1; i <= m; i++)
	{
		int x, y, c;
		fin >> x >> y >> c;
		G[x].push_back({ y, c });
		G[y].push_back({ x, -c });
	}

	bfs(X);

	fout << dist[Y];

	return 0;
}