#include<fstream>
#include<queue>
#include<utility>
#include<vector>
using namespace std;
queue<int>Q;
vector< pair<int,int> >v[30003];
int i,n,m,nod,dist[30003],x,y,a,b,d;
int main()
{
	ifstream fi("sate.in");
	ofstream fo("sate.out");
	fi>>n>>m>>x>>y;
	for(i=1;i<=m;i++)
	{
		fi>>a>>b>>d;
		v[a].push_back(make_pair(b,d));
		v[b].push_back(make_pair(a,-d));
	}
	Q.push(x);
	while(!Q.empty())
	{
		nod=Q.front(); Q.pop();
		if(nod==y) break;
		for(i=0;i<v[nod].size();i++)
		if(!dist[v[nod][i].first] and v[nod][i].first!=x)
		{	dist[v[nod][i].first]=dist[nod]+v[nod][i].second; Q.push(v[nod][i].first); }
		
	}
	fo<<dist[y]<<"\n";
	return 0;
}