#include <fstream>
#include <vector>
#include <queue>
#include <bitset>

using namespace std;

typedef pair <int, int> p;

const int NMAX = 30003;

ifstream fin("sate.in");
ofstream fout("sate.out");

vector <p> g[NMAX];
int dist[NMAX], d;
int n, m, x, y, a, b;

bitset <NMAX> viz;

queue <int> q;

void solve()
{
	while (!q.empty())
	{
		x = q.front();
		if (x == b)
			return;
		viz[x] = 1;
		q.pop();
		for (size_t i = 0; i < g[x].size(); ++i)
		{
			if (viz[g[x][i].first] == 0)
			{
				dist[g[x][i].first] = dist[x] + g[x][i].second;
				q.push(g[x][i].first);
			}
		}
	}
}

int main()
{
	fin >> n >> m >> a >> b;
	for (int i = 1; i <= m; ++i)
	{
		fin >> x >> y >> d;
		g[x].push_back({ y, d });
		g[y].push_back({ x, -d });
	}
	q.push(a);
	solve();
	fout << dist[b] << "\n";
	return 0;
}