#include<fstream>
#include<vector>
#define MAX 30001
using namespace std;
int n,m,x,y,viz[MAX],km[MAX],l[MAX];

struct nod
{
	int vec,cost;
};
vector<nod> a[MAX];
int bfs()
{
	int prim=1,ultim=1,coada[4*MAX],i,ind;
	coada[prim] = x;
	viz[x] = 1;
	while(prim <= ultim)
	{
		ind = coada[prim];
		
		for(i=0;i<l[ind];++i)
		{
			if(!viz[a[ind][i].vec])
			{
				viz[a[ind][i].vec] = 1;
				++ultim;
				coada[ultim] = a[ind][i].vec;
				km[coada[ultim]] = km[ind] + a[ind][i].cost;
			}
		}
		++prim;
	}
	return km[y];
}
int main()
{
	ifstream f("sate.in");
	f>>n>>m>>x>>y;
	int i,j,d,k;
	nod aux;
	for(k=1;k<=n;++k)
	{
		f>>i>>j>>d;
		aux.vec = j;
		aux.cost = d;
		a[i].push_back(aux);
		aux.vec = i;
		aux.cost = -d;
		a[j].push_back(aux);
	}
	f.close();
	
	for(i=1;i<=n;++i)
		l[i] = a[i].size();
	
	ofstream g("sate.out");
	g<<bfs();
	g.close();
	return 0;
}