#include<cstdio>
#include<vector>
#include<queue>
using namespace std;
int n,m,pi,pf,cost[1<<15];
queue<int> Q;
vector<pair<int,int> > L[1<<15];
void citire()
{
	int x=0,y=0,c=0;
	scanf("%d%d%d%d",&n,&m,&pi,&pf);
	for(int i=1;i<=m;i++)
	{
		scanf("%d%d%d",&x,&y,&c);
		L[x].push_back(make_pair(y,c));
		L[y].push_back(make_pair(x,-c));
	}
}
void bfs()
{
	int x=0,nod=0,co=0;
	Q.push(pi);
	cost[pi]=1;
	if(pi==pf) return;
	while(!Q.empty())
	{
		x=Q.front();
		for(vector<pair<int,int> >::iterator it=L[x].begin();it!=L[x].end();it++)
		{
			nod=it->first;
			co=it->second;
			if(cost[nod]==0)
			{
				cost[nod]=cost[x]+co;
				if(nod==pf)
					return;
				Q.push(nod);
			}
		}
		Q.pop();
	}
}
int main()
{
	freopen("sate.in","r",stdin);
	freopen("sate.out","w",stdout);
	citire();
	bfs();
	printf("%d",cost[pf]-1);
	return 0;
}