#include <bits/stdc++.h>
using namespace std;
void hehe() {
  int n;
  cin >> n;
  vector<int> arr(n);
  for (int i = 0; i < n; i++) {
    cin >> arr[i];
  }
  sort(arr.rbegin(), arr.rend());
  vector<long long int> rem;
  for (int i = 0; i < n - 1; i++) {
    if (arr[i] - arr[i + 1] < 2) {
      i++;
      rem.push_back(arr[i]);
    }
  }
  long long int ans = 0;
  for (int i = 0; i + 1 < rem.size(); i += 2) {
    ans += (long long int)(rem[i] * rem[i + 1]);
  }
  cout << ans;
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  int t = 1;
  while (t--) {
    hehe();
  }
  return 0;
}
