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++ (GCC 9.2.1) (judge verdict: Wrong Answer)

#include <bits/stdc++.h>
#include<algorithm>
#define int long long
using namespace std;
typedef pair<int,int> P;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define INF 1000000000000

int MOD=1000000007;
int modpow(int a,int x){
  if(a<0)a+=MOD;
  int ans=1;
  while(x>0){
    if(x&1) ans=ans*a%MOD;
      a=a*a%MOD;
      x>>=1;
  }
  return ans;
}

int gyaku(int n){
    return modpow(n,MOD-2);
}
////////////////////////////////////////////////////////////

signed main() {
  int n;
  cin>>n;
  map<int,int>a;
  rep(i,n){
    int t;
    cin>>t;
    a[t]++;
  }
  int m1=0,m2=0;
  for(auto[i,x]:a){
    if(x>=2){
      if(i>m1){m2=m1;m1=i;}
      else if(i>m2)m2=i;
    }
  }
  cout<<m1*m2<<endl;
}