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

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(uint x, uint y, int c);
    uint BFS(const uint start, const uint dest);
};

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

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

uint Graph::BFS(const uint start, const uint dest)
{
    vector<int> dist(V + 1, 0);
    dist[start] = 1;

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

    while (!Q.empty())
    {
        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);

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

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