Make a Rectangle（Atcoder-2696）

Problem Description

We have N sticks with negligible thickness. The length of the i-th stick is Ai.

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≤N≤105
• 1≤Ai≤109
• Ai is an integer.

Input

Input is given from Standard Input in the following format:

N
A1 A2 ... AN

Output

Print the maximum possible area of the rectangle. If no rectangle can be formed, print 0.

Example

Sample Input 1

6
3 1 2 4 2 1

Sample Output 1

2
1×2 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

题意：给出 n 个棍子的长度，要在这 n 个棍子中选 4 个的棍子组成，求矩形的最大面积

思路：实质是要选两组每组个数为 2 的长度最大的棍子，由于棍子的长度最大到 1E9，无法使用数组进行桶排，因此可以使用 multiset 来进行统计，然后将所有个数大于 2 的棍子加入到优先队列中，取队首前两个元素求积即可

Source Program

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