#include<iostream>
#include<fstream>
#include<vector>
#define f first
#define s second
using namespace std;
int n,m,i,j,bot,top,viz[30002],coada[30002],cost[30002],x,q,a,b,c,y;
vector< pair <int,int> > A[30002];
void bfs()
{
	viz[x]=1;
	bot=0;
	top=1;
	coada[0]=x;
	while(bot<n)
	{
		q=coada[bot];
		for(i=0;i<A[q].size();++i)
			if (!viz[A[q][i].f])
			{
				viz[A[q][i].f] = 1;
				coada[top] = A[q][i].f;
				cost[A[q][i].f] = cost[q] + A[q][i].s;
				top++;
			}
		bot++;
	}
	
	
	
	
}




int main()
{
	ifstream f("sate.in");
	ofstream g("sate.out");
	f>>n>>m>>x>>y;
	for(i=0;i<m;++i)
	{
		f>>a>>b>>c;
		A[a].push_back(make_pair(b,c));
		A[b].push_back(make_pair(a,-c));
	}
	bfs();
	g<<cost[y];
	
}