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

using namespace std;

ifstream f("sate.in");
ofstream g("sate.out");

const int NMAX = 30001;
int n,m,x,y;

struct elem
{
    int x,d;
};

vector <elem> v[NMAX];
queue <int> q;
int d[NMAX];

void citire()
{
    f>>n>>m>>x>>y;

    elem aux; int j; int k;

    for(int i=1; i<=m; i++)
    {
        f>>j>>aux.x>>aux.d;

        v[j].push_back(aux);

        k = aux.x;
        aux.x = j;
        j = k;
        aux.d = -aux.d;

        v[j].push_back(aux);
    }
}

int main()
{
    int aux, i;
    elem s;

    citire();

    q.push(x);
    d[x] = 1;

    while(!q.empty())
    {
        aux = q.front();
        q.pop();

        for(i=0; i<v[aux].size(); i++)
        {
            s = v[aux][i];
            if(d[s.x] == 0)
            {
                d[s.x] = d[aux] + s.d;
                q.push(s.x);
            }
            if(s.x == y)
            {
                while(!q.empty())
                    q.pop();
            }
        }
    }

    g<<d[y] - 1;

    return 0;
}