#include <fstream>
#include <queue>
#include <bitset>
#include <vector>
using namespace std;
string file = "sate";
ifstream cin(file + ".in");
ofstream cout(file + ".out");
vector <int> distanta;
struct sat {
	int nod;
	int distanta;
};
vector <sat> L[30001];

void bfs(int x)
{
	queue <int> Q;
	Q.push(x);
	distanta[x] = 0;
	while (!Q.empty())
	{
		x = Q.front();
		Q.pop();
		for (auto y : L[x])
		{
			if (distanta[y.nod] == -1)
			{
				Q.push(y.nod);
				distanta[y.nod] = distanta[x] + y.distanta;
			}
		}
	}
}
int main()
{
	int n, m, x, y, a,b,d;
	cin >> n >> m >> x >> y;
	distanta.resize(n + 1, -1);
	for (int i = 1; i <= m; i++)
	{
		cin >> a >> b >> d;
		L[a].push_back({b,d});
		L[b].push_back({a,-d});
	}
	bfs(x);
	cout << distanta[y];
}