#include <bits/stdc++.h>
using namespace std;
namespace IO {
void setIn(string s) { freopen(s.c_str(), "r", stdin); }
void setOut(string s) { freopen(s.c_str(), "w", stdout); }
void setIO(string s = "") {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  cin.exceptions(cin.failbit);
  if (s.size()) {
    setIn(s + ".inp");
    setOut(s + ".out");
  } else {
  }
}
}  // namespace IO
using namespace IO;
namespace Function {
template <typename T1, typename T2>
void amax(T1 &a, T2 b) {
  if (typeid(a).name() == typeid(int).name() &&
      typeid(b).name() == typeid(long long).name()) {
    cout << "Error on amax";
    exit(0);
  }
  if (a < b) a = b;
}
template <typename T1, typename T2>
void amin(T1 &a, T2 b) {
  if (a > b) a = b;
}
template <typename T>
void compress(vector<T> &a) {
  sort(a.begin(), a.end());
  a.resize(unique(a.begin(), a.end()) - a.begin());
}
template <typename T>
int get_it(vector<T> a, T b) {
  return lower_bound(a.begin(), a.end(), b) - a.begin();
}
template <typename T1, typename T2>
long long gcd(T1 a, T2 b) {
  return (b == 0 ? a : gcd(b, a % b));
}
template <typename T1, typename T2>
long long lcm(T1 a, T2 b) {
  return 1LL * a / gcd(a, b) * b;
}
}  // namespace Function
using namespace Function;
namespace Output {
void print(int x) { cout << x << "\n"; }
void print(unsigned int x) { cout << x << "\n"; }
void print(long unsigned int x) { cout << x << "\n"; }
void print(long long x) { cout << x << "\n"; }
void print(unsigned long long x) { cout << x << "\n"; }
void print(float x) { cout << x << "\n"; }
void print(double x) { cout << x << "\n"; }
void print(long double x) { cout << x << "\n"; }
void print(char x) { cout << x << "\n"; }
void print(const char *x) { cout << x << "\n"; }
void print(string x) { cout << x << "\n"; }
void print(bool x) { cout << (x ? "True" : "False") << "\n"; }
template <typename T1, typename T2>
void print(pair<T1, T2> a) {
  cout << a.first << " " << a.second << "\n";
}
template <typename T>
void print(T a) {
  for (auto it : a) {
    print(it);
  }
}
template <class T, class... Ts>
void print(T t, Ts... ts) {
  cout << t << " ";
  print(ts...);
}
template <typename T>
void prine(T x) {
  print(x);
  exit(0);
}
template <class T, class... Ts>
void prine(T t, Ts... ts) {
  print(t, ts...);
  exit(0);
}
}  // namespace Output
using namespace Output;
const int dx[] = {1, -1, 0, 0};
const int dy[] = {0, 0, 1, -1};
const int INF = 1e9;
const int MOD = 1e9 + 7;
const int N = 1e5 + 10;
int lab[N], n, m, c[N];
int Get(int u) {
  while (lab[u] > 0) u = lab[u];
  return u;
}
void Union(int u, int v) {
  if (lab[u] > lab[v]) swap(u, v);
  lab[u] += lab[v];
  lab[v] = u;
}
struct Data {
  int u, v, w;
};
bool comp(Data A, Data B) { return A.w > B.w; }
vector<Data> E;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  cin >> n >> m;
  for (int i = 1; i <= n; i++) cin >> c[i];
  for (int i = 1; i <= m; i++) {
    int u, v;
    cin >> u >> v;
    E.push_back({u, v, min(c[u], c[v])});
  }
  sort(E.begin(), E.end(), comp);
  long long sum = 0;
  for (int i = 1; i <= n; i++) lab[i] = -1;
  for (Data X : E) {
    int u = X.u;
    int v = X.v;
    int w = X.w;
    int x = Get(u);
    int y = Get(v);
    if (x == y) continue;
    sum += 1ll * w * lab[x] * lab[y];
    Union(x, y);
  }
  sum *= 2;
  cout << fixed << setprecision(5) << 1.00 * sum / (1ll * n * (n - 1));
}
