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

#define in "sate.in"
#define out "sate.out"
#define N 30005
#define pb push_back
#define y first
#define c second

typedef pair <int, int> nod;
typedef unsigned u;

vector <nod> LIST[N];
vector <int> d (N, 0);
vector <bool> M (N, 0);
queue <int> Q;
int n, m, S, D;

int main ()
{
	ifstream fin (in);
	fin >> n >> m >> S >> D;
	for (int i = 0; i < m; ++i) {
		int x, y, c;
		fin >> x >> y >> c;
		LIST[x].pb (nod (y,c));
		LIST[y].pb (nod (x, -c));
	}
	fin.close();
	Q.push (S);
	M[S] = 1;
	while (Q.size() && !M[D]) {
		int x = Q.front();
		Q.pop();
		for (u i = 0; i < LIST[x].size(); ++i)
			if (!M[LIST[x][i].y]) {
				M[LIST[x][i].y] = 1;
				d[LIST[x][i].y] = d[x] + LIST[x][i].c;
				Q.push (LIST[x][i].y);
			}
	}
	ofstream fout (out);
	fout << d[D];
	fout.close();
	return 0;
}