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| #include<iostream>
#include<cstdio>
#include<cstring>
#include<cctype>
#include<algorithm>
#include<cmath>
using namespace std;
inline int read(){
int w=0,x=0;char c=getchar();
while(!isdigit(c))w|=c=='-',c=getchar();
while(isdigit(c))x=(x<<3)+(x<<1)+(c^48),c=getchar();
return w?-x:x;
}
namespace star
{
const int maxn=1e5+10;
int n,m,siz[maxn],fa[maxn],a[maxn];
double ans;
struct edge{
int u,v,d;
inline bool operator < (const edge &zp) const{return d>zp.d;}
}e[maxn];
inline void onion(int x,int y,int k){
if(siz[x]<siz[y])swap(x,y);
ans+=1.0*siz[x]*siz[y]*k;
fa[y]=x,siz[x]+=siz[y];
}
int find(int x){return fa[x]==x?x:fa[x]=find(fa[x]);}
inline void work(){
n=read(),m=read();
for(int i=1;i<=n;i++) a[i]=read(),fa[i]=i,siz[i]=1;
for(int u,v,i=1;i<=m;i++) u=read(),v=read(),e[i]=(edge){u,v,min(a[u],a[v])};
sort(e+1,e+1+m);
for(int cnt=0,i=1;i<=m and cnt<=n-1;i++){
int u=find(e[i].u),v=find(e[i].v);
if(u!=v)
onion(u,v,e[i].d),cnt++;
}
printf("%.5lf\n",ans/n/(n-1)*2);
}
}
signed main(){
star::work();
return 0;
}
|
