#include <fstream>
#include <vector>
#define NMAx 30100
#define ff first
#define ss second
#define pb push_back
#define mp make_pair
using namespace std;
vector <pair <int,int> > G[NMAx];
int n,nr,X,Y,dist[NMAx],deque[NMAx];

void citire() {
	
	int i,m,x,y,c;
	ifstream in("sate.in");
	in>>n>>m>>X>>Y;
	for(i=0;i<m;i++)
		{in>>x>>y>>c;
		G[x].pb(mp(y,c));
		G[y].pb(mp(x,-c));
		}
	in.close();
	
}
void BFS() {
	
	int i,j,nod;
	
	if(X>Y)
		swap(X,Y);
	deque[1]=X;
	dist[X]=1;
	nr=1;
	
	for(i=1;i<=nr;i++) {
		nod=deque[i];
		for(j=0;j<G[nod].size();j++)
			if(!dist[G[nod][j].ff]) {
				deque[++nr]=G[nod][j].ff;
				dist[G[nod][j].ff]=dist[nod]+G[nod][j].ss;
				if(G[nod][j].ff==Y)
					return;
				}
		}
	
}
int main()
{
	
	citire();
	BFS();
	
	ofstream out("sate.out");
	out<<(dist[Y]-1)<<'\n';
	out.close();
	
	return 0;

}