#include <fstream>
#include <iostream>
using namespace std;

struct nod{
	int capat, cost;
	nod *next;
};

nod * a[30001];
int n, x , y ;


void read(){
	int m ;
	ifstream fin("sate.in");
	fin>>n>>m>>x>>y;
	for(int i=1;i<=n ;i++)
		a[i]= NULL;
	int i,j,c;
	for( ; m ; m--){
		fin >> i >> j >> c;
		nod * tmp = new nod;
		tmp->capat= j;
		tmp->cost= c;
		tmp->next = a[i];
		a[i] = tmp;

		tmp = new nod ;
		tmp -> capat = i;
		tmp -> cost = c;
		tmp -> next = a[j];
		a[j] = tmp;
	}
	fin.close();
}

void write(){
	for(int i =1 ;i<= n;i++){
		nod * p = a[i];
		cout<<i<<" : ";
		for(; p; p=p->next)
			cout<<p->capat<<" ";
		cout<<endl;
	}
}

int bfs(int x, int y){
	int coada [30001], st=1,dr=1, v[30001],cost[30001];
	for(int i =1 ;i<=n;i++)
		v[i]=cost[i]=0;
	coada[1]=x;
	v[x]=1;
	while(st<=dr){
		int k = coada[st++];
		for(nod * p=a[k] ; p ; p = p->next)
			if( v[p->capat] == 0 ){
				int j = p->capat, c = p->cost;
				coada[++dr]	= j;
				v[j] = 1;
				if(k<j)
					cost[j] =  cost[k] + c;
				else
					cost[j] =  cost[k] - c;
				if(j == y){
					//for(int i=1 ; i <= dr;i++)
					//	cout<<coada[i]<<" ";
					//cout<<endl;
					return cost[j];
				}
			}
	}
	return -1;
}

int sol(int x, int y){
	if(x==y)
		return 0;
	if(x>y)
		return bfs(y,x);
	else
		return bfs(x,y);
}

int main(){
	read();
//	cout<<x<<" "<<y<<" "<<sol(x,y);
//	write();
	ofstream fout("sate.out");
	fout<<sol(x,y);
	fout.close();
	return 0;
}