#include <bits/stdc++.h>
using namespace std;
vector<pair<long long, pair<long long, long long> > > edges;
vector<long long> wt(100005);
long long sz[100005], parent[100005];
long long ans = 0;
long long n, m;
void make_set() {
  for (long long i = 1; i <= n; i++) {
    sz[i] = 1;
    parent[i] = i;
  }
}
long long findParent(long long x) {
  if (x == parent[x]) return x;
  return findParent(parent[x]);
}
void merge_sets(long long x, long long y, long long w) {
  x = findParent(x);
  y = findParent(y);
  if (x == y) return;
  ans += w * sz[x] * sz[y];
  if (sz[x] > sz[y]) {
    parent[y] = x;
    sz[x] += sz[y];
  } else {
    parent[x] = y;
    sz[y] += sz[x];
  }
}
int main() {
  cin >> n >> m;
  for (long long i = 1; i <= n; i++) cin >> wt[i];
  for (long long i = 1; i <= m; i++) {
    long long x, y;
    cin >> x >> y;
    edges.push_back(make_pair(min(wt[x], wt[y]), make_pair(x, y)));
  }
  sort(edges.begin(), edges.end());
  reverse(edges.begin(), edges.end());
  make_set();
  for (long long i = 0; i < m; i++) {
    merge_sets(edges[i].second.first, edges[i].second.second, edges[i].first);
  }
  double anss = (double)ans;
  anss /= (double)n;
  anss /= (double)(n - 1);
  anss *= 2.00;
  cout << fixed << setprecision(10) << anss << endl;
  return 0;
}
