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<bits/stdc++.h>
#define all(x) (x).begin(),(x).end()
typedef long long ll;
#define rep(i,n) for(ll i=0, i##_len=(n); i<i##_len; ++i)
#define REP(i,num,n) for(ll i=num, i##_len=(n); i<i##_len; ++i)
template<class T>bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T>bool chmin(T &a, const T &b) { if (b<a) { a=b; return 1; } return 0; }
using namespace std;

int main(void){
    ll N;
    cin >> N;
    ll A;
    map<ll,ll> nums;
    rep(i,N) {
      cin >> A;
      ++nums[A];
    }
    ll side = 0;
    ll side2 = 0;
    ll ans = 0;
    for(auto itr = nums.begin(); itr != nums.end(); ++itr){
        if(side != 0 && itr->second >= 2){
          side2 = itr->first;
          chmax(ans, side * side2);
          side = max(side, side2);
          if(itr->second >= 4) chmax(ans, side2 * side2);
          side2 = 0;
        }
        else if(itr->second >= 2){
           side = itr->first;
           if(itr->second >= 4) chmax(ans, side * side);
        }
    }
    cout << ans << endl;
}