#include <cstdio>
#include <vector>
#include <queue>
using namespace std;

struct muchie {
	int sat,dist;
	
};
vector <muchie> v[30005];
queue <int> q;
int dist[30005];
int n,m,x,y;

muchie mk_muchie(int a,int d) {
	muchie res;
	res.sat = a;
	res.dist = d;
	return res;
}

inline void bfs() {
	dist[x] = 1;
	q.push(x);
	while (!q.empty() && dist[y] == 0) {
		int n = q.front(); q.pop();
		while (!v[n].empty()) {
			muchie crt = v[n].back(); v[n].pop_back();
			int nod = crt.sat;
			if (dist[nod] > 0) continue;
			if (nod > n) dist[nod] = dist[n] + crt.dist;
			else dist[nod] = dist[n] - crt.dist;
			q.push(nod);
		}
	}
}

int main() {
	freopen("sate.in","r",stdin);
	freopen("sate.out","w",stdout);
	scanf("%d %d %d %d",&n,&m,&x,&y);
	for (int i=1;i<=m;i++) {
		int a,b,d;
		scanf("%d %d %d",&a,&b,&d);
		v[a].push_back(mk_muchie(b,d));
		v[b].push_back(mk_muchie(a,d));
	}
	bfs();
	printf("%d\n",dist[y]-1);
	return 0;
}