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++14 (GCC 5.4.1) (judge verdict: Runtime Error)

#include<bits/stdc++.h>

#define REP(i,a,b) for(int i=a,i##_end_=b;i<=i##_end_;++i)
#define DREP(i,a,b) for(int i=a,i##_end_=b;i>=i##_end_;--i)
#define MREP(i,x) for(int i=beg[x],v;v=to[i],i;i=las[i])
#define debug(x) cout<<#x<<"="<<x<<endl
#define fi first
#define se second
#define mk make_pair
#define pb push_back
#define y1 asdasd
typedef long long ll;

using namespace std;

void File(){
	freopen("speech.in","r",stdin);
	freopen("speech.out","w",stdout);
}

template<typename T>void read(T &_){
	T __=0,mul=1; char ch=getchar();
	while(!isdigit(ch)){
		if(ch=='-')mul=-1;
		ch=getchar();
	}
	while(isdigit(ch))__=(__<<1)+(__<<3)+(ch^'0'),ch=getchar();
	_=__*mul;
}

const int maxn=2000+10;
int n,m,a[maxn][maxn],b[maxn][maxn],ans;
char s[maxn];
stack<int>stk;

int main(){
	//File();
	read(n); read(m);
	if(n==1 || m==1)return printf("%d\n",n*m),0;
	REP(i,1,n){
		scanf("%s",s+1);
		REP(j,1,m)a[i][j]=(s[j]=='#');
	}
	REP(i,1,n-1)REP(j,1,m-1){
		int c[2]={0};
		++c[a[i][j]];
		++c[a[i+1][j]];
		++c[a[i][j+1]];
		++c[a[i+1][j+1]];
		b[i][j]=!(c[1]%2);
		if(b[i][j])b[i][j]+=b[i-1][j];
	}
	/*REP(i,1,n-1){
		REP(j,1,n-1)
			cout<<b[i][j]<<" ";
		cout<<endl;
	}*/
	REP(i,1,n-1){
		int las;
		REP(j,1,m-1){
			las=j;
			while(!stk.empty() && b[i][stk.top()]>=b[i][j]){
				las=stk.top(); stk.pop();
				if(b[i][las])ans=max(ans,(j-las+1)*(b[i][las]+1));
			}
			b[i][las]=b[i][j];
			stk.push(las);
		}
		while(!stk.empty()){
			las=stk.top(); stk.pop();
			if(b[i][las])ans=max(ans,(n+1-las+1)*(b[i][las]+1));
		}
	}
	printf("%d\n",max(ans,max(n,m)));
	return 0;
}