#include<cstdio>
#include<vector>
#include<queue>
#define NMAX 30005
using namespace std;
int p,u,n,m,x,y,q,w,d,viz[NMAX],C[NMAX],dist[NMAX],i,nod;
vector<int >G[NMAX],K[NMAX];
queue<int>Q;
int main (){
	
	freopen("sate.in","r",stdin);
	freopen("sate.out","w",stdout);
	
	scanf("%d%d%d%d",&n,&m,&x,&y);
	
	for(i=1;i<=m;i++){
		
		scanf("%d%d%d",&q,&w,&d);
		
		G[q].push_back(w);
		G[w].push_back(q);
		
		if(q>w){
			K[q].push_back(-d);
			K[w].push_back(d);
		}
		else{
			K[q].push_back(d);
			K[w].push_back(-d);
		}
	}
	
	viz[x]=1;
	Q.push(x);
	bool ok=1;
	while(!Q.empty() && ok){
		nod=Q.front();
		Q.pop();
		for(int i=0;i<G[nod].size();++i){
			
			if(!viz[G[nod][i]]){
				
				viz[G[nod][i]]=1;
				dist[G[nod][i]]+=dist[nod]+K[nod][i];
				Q.push(G[nod][i]);
				
			}
			if(G[nod][i]==y)
				
				ok=1;
		}
	}
	printf("%d",dist[y]);
	return 0;
}