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: Wrong Answer)

#include <bits/stdc++.h>
using namespace std;

int main() {
  int n;
  cin >>n;
  vector<int>a(n);
  for(int i=0;i<n;i++){
    cin>>a[i];
    a[i]*=-1;
  }
  map<int,int>mp;
  for(int x:a){
    if(mp.count(x))mp.at(x)++;
    else mp[x]=1;
  }
  int tate=0;
  int yoko=0;
  long long kotae=0;
  auto begin=mp.begin(),end=mp.end();
  for(auto iter=begin;iter!=end;iter++){
    if(iter->second >=2 && tate==0){
      tate=iter->first;
      mp[iter->first]-=2;
    }
    if(iter->second >=2 && tate!=0 && yoko==0){
      yoko=iter->first;
    }
    if(tate!=0&&yoko!=0)break;
  }
  kotae=tate*yoko;
  cout<<kotae<<endl; 
  return 0;
}