#include <stdio.h>
#include <vector>
#include <queue>
#define N 1<<15
#define mp make_pair
#define pb push_back
#define f first
#define s second
using namespace std;
int n,m,x,y;
int d[N];
char marc[N];
vector < pair <int, int> > v[N];
queue <int> Q; 
void read()
{
	scanf("%d%d%d%d",&n,&m,&x,&y);
	int i,a,b,c;
	for (i=1; i<=m; i++)
	{
		scanf("%d%d%d",&a,&b,&c);
		v[a].pb(mp(b,c));
		v[b].pb(mp(a,c));
	}
}
void solve()
{
	int i,t;
	Q.push(x);
	marc[x]=1;
	while (!Q.empty())
	{
		t=Q.front();
		Q.pop();
		for (i=0; i<v[t].size(); i++)
			if (!marc[v[t][i].f])
			{
				if (v[t][i].f<t)
					d[v[t][i].f]=v[t][i].second+d[t];
				else
					d[v[t][i].f]=d[t]-v[t][i].second;
				Q.push(v[t][i].f);
				marc[v[t][i].f]=1;
			}
	}
}
int main()
{
	freopen("sate.in","r",stdin);
	freopen("sate.out","w",stdout);
	read();
	solve();
	if (!marc[y])
		printf("-1\n");
	else
		printf("%d\n",-d[y]);
	return 0;
}