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 5.4.1) (judge verdict: Wrong Answer)

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#include<math.h>

typedef long long int ll;

int MAX(int a, int b){return a>b?a:b;}
int MIN(int a, int b){return a>b?b:a;}
int compare(const void* a, const void* b){return *(int*)a - *(int*)b;}
ll llMAX(ll a, ll b){return a>b?a:b;}
ll llMIN(ll a, ll b){return a>b?b:a;}
ll digits(ll a){ll tmp=a; ll cnt=0; while(tmp>0){tmp/=10; cnt+=1;} return cnt;}

int main(void){
    int n;
    scanf("%d", &n);
    int a[n];
    for(int i=0; i<n; i++){scanf("%d", &a[i]);}

    qsort(a, n, sizeof(int), compare);

    int b_idx = 0;
    int b[n];// [][0]:長さ
    int c_idx = 0;
    int c[n];
    for(int i=0; i<n; i++){b[i]=0; c[i]=0;}

    int cnt = 0;
    int tmp = a[n-1];
    for(int i=n-1; i>=0; i--){
        if(tmp == a[i]){
            cnt += 1;
        }else{
            if(cnt > 3){
                c[c_idx] = tmp;
                c_idx +=1 ;
            }else if(cnt > 1){
                b[b_idx] = tmp;
                b_idx += 1;
            }
            tmp = a[i];
            cnt = 1;
        }
    }

    if(cnt > 3){
        c[c_idx] = tmp;
    }
    else if(cnt > 1){
        b[b_idx] = tmp;
    }

    ll ans = 0;

    for(int i=0; i<c_idx; i++){
        ans = llMAX(c[i]*c[i], ans);
    }

    for(int i=0; i<b_idx; i++){
        ans = llMAX(b[i]*b[i+1], ans);
    }

    printf("%lld\n", ans);

    return 0;
}