#include <fstream>
#include <queue>
#include <vector>
#define MAX_SATE 30001
using namespace std;

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

struct sat
{
    int id,d;
};

int n,m,x,y;
int dist[MAX_SATE];
queue <int> C;
vector <sat> G[MAX_SATE];

void BF()
{
    int vec;
    C.push(x);
    dist[x] = 1;

    while(!C.empty())
    {
        vec = C.front();

        for(unsigned j=0;j<G[vec].size();++j)
        {
            if(!dist[G[vec][j].id])
            {
                dist[G[vec][j].id] = dist[vec] + G[vec][j].d;
                if(G[vec][j].id==y) return;
                C.push(G[vec][j].id);
            }
        }
        C.pop();
    }
}
int main()
{
    int i,j,d;
    fin>>n>>m>>x>>y;

    for(int k=0;k<m;++k)
    {
        fin>>i>>j>>d;

        sat p,q;

        p.id = i;
        p.d = -d;

        q.id = j;
        q.d = d;

        G[i].push_back(q);
        G[j].push_back(p);

    }

    if(x!=y) BF();

    fout<<dist[y]-1;

    return 0;
}