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

ifstream cin("sate.in");
ofstream cout("sate.out");

vector<vector<pair<int, int> > >lis;
int n, m, x, y;long long  viz[30005],uz[30005];

void read();
void solve();

int main()
{
	read();
	solve();
	cout << viz[y]-1 << '\n';
	cin.close(); cout.close();
	return 0;
}

vector<pair<int,int> > gol;

void read()
{
	cin >> n >> m >> x >> y;
	for (int i = 0; i <= n; i++) lis.push_back(gol);
	for (int i = 1; i <= m; i++)
	{
		int a, b, c;
		cin >> a >> b >> c;
		lis[a].push_back(make_pair(b, c));
		lis[b].push_back(make_pair(a, -c));
	}
}

void solve()
{
	queue<int> que;
	que.push(x); viz[x] = 1;// uz[x] = 1;
	while (!que.empty())
	{
		int nod = que.front(); que.pop();
		for (int i = 0; i < lis[nod].size(); i++)
		{
			int nod2 = lis[nod][i].first;
			if (viz[nod2] == 0)
			{
				//uz[nod2] = 1;
				viz[nod2] = viz[nod] + lis[nod][i].second;
				que.push(nod2);
			}
		}
	}
}