#include<fstream>
#include<vector>
#include<queue>
#define nmn 30002
using namespace std;
vector<int> v[nmn];
vector<int> c[nmn];
//queue<int> q;
int n,m,X,Y;
int d[nmn];
void citire()
{
	int i,x,y,cost;
	ifstream in("sate.in");
	in>>n>>m>>X>>Y;
	for (i=1;i<=m;i++)
	{
		in>>x>>y>>cost;
		v[x].push_back(y);
		v[y].push_back(x);
		c[x].push_back(cost);
		c[y].push_back(-cost);
	}
}
void afisare()
{
	ofstream out("sate.out");
	out<<d[Y]-1;
}
int main()
{
	int i,x,y,ok=1,prim=1,ultim=1,q[100100],lng;
	citire();
	q[1]=X;
	d[X]=1;	
	while (prim<=ultim)
	{
		x=q[prim];
		prim++;
		lng=(int)v[x].size();
		for (i=0;i<lng;i++)
		{
			y=v[x][i];
			if (y!=X&&!d[y])
			{
				d[y]=d[x]+c[x][i];
				q[++ultim]=y;			}
			if (d[Y])
			{
				afisare();
				return 0;
			}
		}
	}	
	afisare();
	
}