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 Python (3.4.3) (judge verdict: Accepted)

from collections import Counter

N = int(input())
A = list(map(int, input().split()))
c_dict = Counter(A)
B = []
C = []

for k, v in c_dict.items():
    if v >= 4:
        B.append(k)
    elif v >= 2:
        C.append(k)
B.sort(reverse=True)
C.sort(reverse=True)

if len(B) < 1 and len(C) < 2:
    print(0)
elif len(B) < 1 and len(C) >= 2:
    F = C[1] * C[0]
    print(F)
elif len(B) >= 1 and len(C) == 1:
    D = B[0] ** 2
    E = C[0] * B[0]
    print(max(D, E))
elif len(B) >= 1 and len(C) >= 2:
    D = B[0] ** 2
    E = C[0] * B[0]
    F = C[1] * C[0]
    print(max(D, E, F))