#include <fstream>

using namespace std;
ifstream in("sate.in");
ofstream out("sate.out");
int a[100025],b[100025],dist[100025],rasp[30005];
int main()
{
    int n,m,x,y,i;
    in>>n>>m>>x>>y;
    rasp[x]=1;
    for(i=1;i<=m;i++){
        in>>a[i]>>b[i]>>dist[i];
        if(rasp[a[i]] and rasp[b[i]]==0)
             rasp[b[i]]=rasp[a[i]]+dist[i];
    else if(rasp[a[i]]==0 and rasp[b[i]])
             rasp[a[i]]=rasp[b[i]]-dist[i];
    }
    while(!rasp[y])
    for(i=1;i<=m;i++){
        if(rasp[a[i]] and rasp[b[i]]==0)
             rasp[b[i]]=rasp[a[i]]+dist[i];
    else if(rasp[a[i]]==0 and rasp[b[i]])
             rasp[a[i]]=rasp[b[i]]-dist[i];
    }
    out<<rasp[y]-1;
    return 0;
}