#include <fstream>
#include <vector>
#include <cstdlib>

typedef std::pair<int,int> PII;

int main(){
	std::ifstream fin("sate.in");
	std::ofstream fout("sate.out");

	int n,m,x,y;
	fin>>n>>m>>x>>y;

	std::vector< std::vector<PII> > Adj(n+1);

	for(int i=0;i<m;++i){
		int a,b,c; fin>>a>>b>>c;
		Adj[a].push_back(PII(b,c));
		Adj[b].push_back(PII(a,c));
	}

	std::vector<int> dist(n+1,-1);

	std::vector<int> q(n);
	q[0]=x; dist[x]=0;
	unsigned left=0,right=1;

	if(x==y){ fout<<"0\n"; return 0; } //ar fi prostie totusi :)

	bool found=false;
	while(!found){
		int t=q[left++];

		for(auto it=Adj[t].begin();it!=Adj[t].end();++it)
			if(dist[it->first]==-1){
				int semn=1;
				if(it->first<t) semn*=-1;
				if(x>t) semn*=-1;
				dist[it->first]=std::abs(dist[t]+semn*it->second);
				if(it->first==y) found=true;
				else q[right++]=it->first;
			}
	}
	fout<<dist[y]<<'\n';//*/
}