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

int find_dist(const vector<vector<pair<int, int> > >& graf,
	const int x, const int y){
	vector<int> dist(graf.size(), -1);
	dist[x] = 0;
	queue<int> de_viz;
	de_viz.push(x);
	while(dist[y] == -1){
		const int cur = de_viz.front();
		de_viz.pop();
		for(const auto next : graf[cur]){
			if(dist[next.first] == -1){
				dist[next.first] = dist[cur] + next.second;
				de_viz.push(next.first); } } }
	return dist[y]; }

int main(){
	ifstream f("sate.in");
	ofstream g("sate.out");
	int n, m, x, y;
	f >> n >> m >> x >> y;
	--x, --y;
	vector<vector<pair<int, int> > > v(n);
	for(int i = 0, a, b, c; i < m; ++i){
		f >> a >> b >> c;
		--a, --b;
		v[a].emplace_back(b, c);
		v[b].emplace_back(a, -c); }
	g << find_dist(v, x, y);
	return 0; }