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

using namespace std;
typedef unsigned int uint;

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

class Graph
{
	struct cost
	{
		int node,weight;
	};
	uint V;
	vector<vector<cost>> adj;
public:
	Graph(const uint V);
	void Add_edge(int x, int y, int c);
	uint BFS(const uint start, const uint dest);
};

inline Graph::Graph(const uint V)
{
	this->V = V;
	adj.assign(V + 1, vector<cost>());
}

inline void Graph::Add_edge(int x, int y, int c)
{
	adj[x].push_back({y,c});
	adj[y].push_back({x,-c});
}

inline uint Graph::BFS(const uint start, const uint dest)
{
	int dist[V + 1];
	memset(dist, 0, sizeof(dist));
    dist[start] = 1;

	queue<uint> Q;
	Q.push(start);

	while (!dist[dest])
	{
		uint n = Q.front();
		Q.pop();

		for (auto &i : adj[n])
		{
			if (!dist[i.node])
			{
				dist[i.node] = i.weight + dist[n];
				Q.push(i.node);
			}
		}
	}

	return dist[dest] - 1;
}

int main()
{
	uint n,m,src,dest;
	fin >> n >> m >> src >> dest;

	Graph G(n);
	{
	    int x,y,c;
		for (uint i = 0; i < m; ++i)
		{
			fin >> x >> y >> c;
			G.Add_edge(x,y,c);
		}
	}

	fout << G.BFS(src,dest);
}