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 Pascal (FPC 2.6.2) (judge verdict: Accepted)

var n:longint;
    i:longint;
    a,num:array[0..500000]of int64;
    max,max2,x:int64;
procedure qs(l,r:longint);
var i,j,m,t:longint;
begin
  i:=l;
  j:=r;
  m:=a[(l+r)>>1];
  repeat
    while a[i]>m do inc(i);
    while a[j]<m do dec(j);
    if i<=j then
    begin
      t:=a[i];a[i]:=a[j];a[j]:=t;
      inc(i);
      dec(j);
    end;
  until i>j;
  if l<j then qs(l,j);
  if i<r then qs(i,r);
end;
begin
  read(n);
  for i:=1 to n do
  begin
    read(a[i]);
    num[i]:=1;
  end;
  qs(1,n);
  for i:=1 to n do
  if a[i]=a[i-1] then inc(num[i],num[i-1]);
  for i:=1 to n do
  if num[i]>=4 then
  begin
    if x=0 then x:=a[i]*a[i];
  end else
  if num[i]>=2 then
  begin
    if max=0 then max:=a[i] else
    if (max2=0)and(a[i]<>max) then max2:=a[i];
  end;
  if x>max*max2 then writeln(x) else
  writeln(max*max2);
end.