#include<fstream>
using namespace std;
int a,b,sol[30001],x[100001],y[100001],c[100001],N,M;
int main()
{
	int i;
	ifstream in("sate.in");
	ofstream out("sate.out");
	in>>N>>M>>a>>b;
	sol[a]=1;
	for (i=1;i<=M;i++)
		in>>x[i]>>y[i]>>c[i];
	while (!sol[b])
	{
		for (i=1;i<=M;i++)
		{
			if (sol[x[i]]&&!sol[y[i]])
				sol[y[i]]=sol[x[i]]+c[i];
			else if (!sol[x[i]]&&sol[y[i]])
				sol[x[i]]=sol[y[i]]-c[i];
		}
	}
	out<<sol[b]-1;
}