#include <bits/stdc++.h>
#define MAXN 30000
#define pii pair<int,int>
#define v vector<pii>
#define F first
#define S second
using namespace std;

int V[MAXN],n,m,x,y;
v A[MAXN];

ifstream fin("sate.in");
ofstream fout("sate.out");

int BFS(int x, int y){
	queue <int> q;
	q.push(y);
	while(!q.empty()){
		int f=q.front(),l=A[f].size();
		V[f]=-1;
		if(f==x){
			while(!q.empty())q.pop();
			break;
		} 
		for(int i=0;i<l;i++){
			if(!V[A[f][i].F])q.push(A[f][i].F);
		}
		q.pop();
	}
	q.push(x);
	V[x]=0;
	while(!q.empty()){
		int f=q.front(),l=A[f].size();
		if(f==y)break; 
		for(int i=0;i<l;i++){
			if(V[A[f][i].F]==-1){
				int node=A[f][i].F;
				q.push(node);
				if(f<node)V[node]=V[f]+A[f][i].S;
				else V[node]=V[f]-A[f][i].S;
			}
		}
		q.pop();
	}
	return V[y];
}

int main(){
	fin>>n>>m>>x>>y;
	memset(V,0,n+1);
	for(int i=0;i<m;i++){
		int a;
		pii p;
		fin>>a>>p.F>>p.S;
		A[a].push_back(p);
		swap(a,p.F);
		A[a].push_back(p);
	}
	fout<<BFS(x,y);
	return 0;
}