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

bitset<36001> v;
vector<pair<int,int>> g[30001];
int d[30001], n, m, x, y,a,b,c;
int bfs(int& x,int& y){
	queue<int> q;
	q.push(x);
	while (!q.empty()){
		int r = q.front();
		q.pop();
		for (auto& i : g[r]){
			if (r == y) return r < y ? d[r] + i.second : d[r] - i.second;
			else if (!v[i.first]){
				if (r < i.first) d[i.first] = d[r] + i.second;
				else d[i.first] = d[r] - i.second;
				v[i.first] = 1;
				q.push(i.first);
			}
		}
	}
}

int main(){
	ifstream f("sate.in");
	ofstream of("sate.out");
	f >> n >> m >> x >> y;
	for (int i = 0; i < m; ++i){
		f >> a >> b >> c;
		g[a].push_back(make_pair(b, c));
		g[b].push_back(make_pair(a, c));
	}
	memset(d, -1, sizeof(d));
	d[x] = 0;
	bfs(x, y);
	of << d[y];
}