#include<fstream.h>
#include<vector>
using namespace std;
const int N=30001;
vector<int> v1[N];
vector<int> v2[N];
int n,m,x,y,dist[N],e[N];

ifstream in("sate.in");
ofstream out("sate.out");

void bfs(int s)
{
	int st,dr,i;
	st=dr=1;
	dist[st]=s;
	e[s]=0;
	while(st<=dr)
	{
		for(i=0;i<v1[dist[st]].size();i++)
		{
			if(e[v1[dist[st]][i]]==-1)
			{
				dist[++dr]=v1[dist[st]][i];
				e[v1[dist[st]][i]]=e[dist[st]]+v2[dist[st]][i];
			}
		}
		st++;
	}
}
int main()
{
	//freopen("sate.in","r",stdin);
	//freopen("sate.out","w",stdout);
	int i,a,b,s;
	//scanf("%d%d%d%d",&n,&m,&x,&y);
	in>>n>>m>>x>>y;
	for(i=1;i<=m;i++)
	{
		//scanf("%d%d%d",&a,&b,&s);
		in>>a>>b>>s;
		v1[a].push_back(b);
		v1[b].push_back(a);
		v2[a].push_back(s);
		v2[b].push_back(-s);
	}
	memset(e,-1,sizeof(e));
	bfs(x);
	//printf("%d",e[y]);
	out<<e[y];
	return 0;
}