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: Memory Limit Exceeded)

var
        a:array[0..1000000000]of byte;
        i,n,c,max,m:longint;
        flag:boolean;
begin
        fillchar(a,sizeof(a),0);max:=0;
        read(n);
        for i:=1 to n do
        begin
                read(c);
                if c>max then max:=c;
                if (a[c]<=4)then inc(a[c]);
        end;
        flag:=true;m:=0;
        for i:=max downto 1 do
        begin
                if (a[i]>=4)and(m=0)then begin writeln(i*i);halt;end;
                if (a[i]>=2) then if m=0 then m:=i
                                         else begin writeln(m*i);halt;end;
        end;
        writeln(0);
end.