Problem:
Score : 300 points

Problem Statement

We have N sticks with negligible thickness.
The length of the i-th stick is A_i.

Snuke wants to select four different sticks from these sticks and form a rectangle (including a square), using the sticks as its sides.
Find the maximum possible area of the rectangle.

Constraints

4 \leq N \leq 10^5

1 \leq A_i \leq 10^9

A_i is an integer.

Input

Input is given from Standard Input in the following format:

N
A_1 A_2 ... A_N

Output

Print the maximum possible area of the rectangle.
If no rectangle can be formed, print 0.

Sample Input 1

6
3 1 2 4 2 1

Sample Output 1

2

1 \times 2 rectangle can be formed.

Sample Input 2

4
1 2 3 4

Sample Output 2

0

No rectangle can be formed.

Sample Input 3

10
3 3 3 3 4 4 4 5 5 5

Sample Output 3

20

Solution in C++14 (GCC 5.4.1) (judge verdict: Accepted)

#include <cstdio>
#include <iostream>
#include <vector>
#include <map>
#include <set>
#include <deque>
#include <queue>
#include <string>
#include <algorithm>
#include <bitset>
using namespace std;

typedef long long ll;

//#define dbg(arg) (printf arg)
#define dbg(arg)

ll find_same(vector<ll> &es, vector<ll>::iterator &it) {
  if (it == es.end()) return 0LL;
  ll num = *it;
//  cout << num << endl;
  while (it != es.end()) {
    ++it;
    if (num == (*it)) {
      ++it;
      return num;
    } else {
      num = (*it);
    }
  }
  return 0LL;
}

int main() {
    int n;
    scanf("%d", &n);
    vector<ll> es;
    for (int i = 0; i < n; ++i) {
      ll e;
      scanf("%lld", &e);
      es.emplace_back(e);
    }
    sort(es.begin(), es.end(), greater<ll>());
    auto it = es.begin();
    ll l[2] = { 0, 0 };;
    for (int i = 0; i < 2; ++i) {
      l[i] = find_same(es, it);
//      cout << it - es.begin() << ": " << l[i] << endl;
    }
    printf("%lld\n", l[0] * l[1]);
    return 0;
}