DSU

//http://codeforces.com/problemset/problem/437/D
#include<bits/stdc++.h>
using namespace std;

#define si(n) scanf("%d",&n)
#define f first
#define s second
#define mp(a,b) make_pair(a,b)
#define MAX 100005

int n,m,par[MAX];

int root(int v){return par[v]<0?v:(par[v]=root(par[v]));}

void union_set(int u,int v)
{
    if((u=root(u))==(v=root(v)))return ;
    if(par[u]>par[v])swap(u,v);
    par[u]+=par[v];
    par[v]=u;
}


vector<int>gr[MAX];
int vis[MAX];

int main()
{
    int i,j;
    si(n);si(m);
    memset(par,-1,sizeof(par));
    vector<pair<int,int> >all;

    for(i=0;i<n;i++){
        int w;
        si(w);
        all.push_back(mp(w,i));
    }
    sort(all.begin(),all.end());

    for(i=0;i<m;i++){
        int u,v;
        si(u);si(v);u--;v--;
        gr[u].push_back(v);
        gr[v].push_back(u);
    }
    long long ans=0;

    for(i=all.size()-1;i>=0;i--){
        long long t=0;
        int nd=all[i].s,val=all[i].f;
        for(j=0;j<gr[nd].size();j++){
            int to=gr[nd][j];
            if(!vis[to])continue;
            int now=root(nd);
            to=root(to);
            if(now==to)continue;
            t+=(long long)(-par[now])*(-par[to]);
            union_set(now,to);
        }
        t*=val;
        ans+=t;
        vis[nd]=1;
    }
    ans*=2;
    printf("%.6f\n",(double)ans/((long long)n*(n-1)));
}