#include <fstream>
#include <vector>
using namespace std;
struct elem{
	int y, c;
};
vector <elem> v[30005];
int f[30005];
void dfs( int x ){
	int i, y, c;
	for( i = 0; i < v[x].size(); i++ ){
		y = v[x][i].y;
		c = v[x][i].c;
		if( f[y] == 0 ){
			f[y] = f[x] + c;
			dfs( y );
		}
	}
}
int main(){
	int n, m, a, b, i, x, y, c;
	ifstream fin( "sate.in" );
	ofstream fout( "sate.out" );
	fin >> n >> m >> a >> b;
	for( i = 0; i < m; i++ ){
		fin >> x >> y >> c;
		if( x > y ){
			swap( x, y );
		}
		v[x].push_back( { y, c } );
		v[y].push_back( { x, -c } );
	}
	f[a] = 1;
	dfs( a );
	fout << f[b] - 1;
	return 0;
}