#include <bits/stdc++.h>
using namespace std;
template <typename T1, typename T2>
string print_iterable(T1 begin_iter, T2 end_iter, int counter) {
  bool done_something = false;
  stringstream res;
  res << "[";
  for (; begin_iter != end_iter and counter; ++begin_iter) {
    done_something = true;
    counter--;
    res << *begin_iter << ", ";
  }
  string str = res.str();
  if (done_something) {
    str.pop_back();
    str.pop_back();
  }
  str += "]";
  return str;
}
vector<int> SortIndex(int size, std::function<bool(int, int)> compare) {
  vector<int> ord(size);
  for (int i = 0; i < size; i++) ord[i] = i;
  sort(ord.begin(), ord.end(), compare);
  return ord;
}
template <typename T>
bool MinPlace(T& a, const T& b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <typename T>
bool MaxPlace(T& a, const T& b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <typename Container>
int SZ(const Container& S) {
  return S.size();
}
template <typename S, typename T>
ostream& operator<<(ostream& out, const pair<S, T>& p) {
  out << "{" << p.first << ", " << p.second << "}";
  return out;
}
template <typename T>
ostream& operator<<(ostream& out, const vector<T>& v) {
  out << "[";
  for (int i = 0; i < (int)v.size(); i++) {
    out << v[i];
    if (i != (int)v.size() - 1) out << ", ";
  }
  out << "]";
  return out;
}
const int MAXN = 100100;
int pp[MAXN];
int ss[MAXN];
int boss(int v) {
  if (pp[v] != v) pp[v] = boss(pp[v]);
  return pp[v];
}
long long Join(int a, int b) {
  a = boss(a);
  b = boss(b);
  if (a == b) return 0;
  if (ss[a] < ss[b]) swap(a, b);
  long long res = ((long long)ss[a]) * ss[b];
  ss[a] += ss[b];
  pp[b] = a;
  return res;
}
vector<int> aa[MAXN];
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  int N, M;
  cin >> N >> M;
  for (int i = 1; i <= N; i++) pp[i] = i, ss[i] = 1;
  vector<long long> val(N + 1);
  for (int i = 1; i <= N; i++) cin >> val[i];
  for (int i = 0; i < M; i++) {
    int a, b;
    cin >> a >> b;
    aa[a].push_back(b);
    aa[b].push_back(a);
  }
  vector<int> ord(N);
  for (int i = 0; i < N; i++) ord[i] = i + 1;
  sort(ord.begin(), ord.end(), [&](int i, int j) { return val[i] > val[j]; });
  long long res = 0;
  for (int v : ord) {
    for (int a : aa[v]) {
      if (val[a] < val[v]) continue;
      res += val[v] * Join(v, a);
    }
  }
  res *= 2;
  cout.precision(20);
  cout << fixed << ((double)res) / (((long long)N) * (N - 1)) << "\n";
}
