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

using namespace std;
typedef unsigned int uint;

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

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

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

uint BFS(const uint V, 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;

	adj.assign(n + 1, vector<cost>());

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

	fout << BFS(n,src,dest);
}