#include <bits/stdc++.h>
using namespace std;
ifstream fin("sate.in");
ofstream fout("sate.out");
vector<pair<int, int>>a[30002];
int n, m, X, Y;
/// d[i]-distanta de la X la i
/// v[i]=1 daca s a gasit gasit distanta de la X la i
int d[30002],v[30002];
queue<int>Q;
int main()
{
	int i,x,y,dist;
	fin >> n >> m >> X >> Y;
	for (i = 1; i <= m; i++)
	{
		fin >> x >> y >> dist;
		a[x].push_back({ y,dist });
		a[y].push_back({ x,-dist });
	}
	d[X] = 0;
	v[X] = 1;
	Q.push(X);
	while (!Q.empty())
	{
		x = Q.front();
		Q.pop();
		for (auto w : a[x])
		{
			y = w.first;
			dist = w.second;
			if (v[y] == 0)
			{
				v[y] = 1;
				d[y] = d[x] + dist;
				if (y == Y) {
					fout << d[y]; return 0;
				}
				Q.push(y);
			}
		}
	}
	return 0;
}