#include <bits/stdc++.h>
using namespace std;
template <typename T, typename U>
std::pair<T, U> operator+(const std::pair<T, U>& l, const std::pair<T, U>& r) {
  return {l.first + r.first, l.second + r.second};
}
typedef void (*callback_function)(void);
const long long INF64 = 1e18;
const long long INF32 = 1e9;
const long long MOD = 1e9 + 7;
const long double PI = acos(-1.0L);
const long double EPS = static_cast<long double>(1e-9);
void input() {}
void preprocess() {}
void solve() {
  preprocess();
  long long n;
  cin >> n;
  long long a[n];
  for (long long i = (0); i < (n); i++) cin >> a[i];
  sort(a, a + n, greater<long long>());
  vector<long long> res;
  for (long long i = (1); i < (n); i++) {
    if (a[i - 1] - a[i] <= 1) {
      res.push_back(min(a[i - 1], a[i]));
      i++;
    }
  }
  long long ans = 0;
  for (long long i = (0); i < (res.size() / 2); i++) {
    ans += res[i * 2] * res[i * 2 + 1];
  }
  cout << ans << endl;
}
void output() {}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie();
  {
    input();
    solve();
    output();
  }
  return 0;
}
