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: Runtime Error)

#include <vector>
#include <iostream>
using namespace std;

using uint = unsigned int;
using ll = long long;
using ull = unsigned long long;
template <class T> using V = vector<T>;
template <class T> using VV = V<V<T>>;
#define rep(i, n) for(int i = 0; i<n; i++)
#define rep_s(i, start, n) for(int i= start, i<n, i++)
#define rep_down(i, n) for(int i = n-1; i>=0; i--)
#define rep_s_down(i, start, n) for(int i = n-1; i>=start; i--)

int main(void){
    ll N;
  	cin >> N;
  	ll a[N];
  	int zyougen = (1e+5)+1; 
  	int count[zyougen]={};
  	
  	rep(i, N) {
      cin >> a[i];
      count[a[i]]++;
    }
  	ll ans=1; 
  	int count_num = 0;

  	rep_s_down(i,1, zyougen){
      if (count[i]>=2) {
        ans = ans * i;
        count_num++;
      }
      if(count_num == 2) {
        cout << ans << endl;
        break;
      }
      if (i == 1) {
         cout << 0 << endl;
      }
    }
}