# include <bits/stdc++.h>

using namespace std;

char arr[15][15];
long long n,m;

long long func(int x1,int y1,int x2,int y2)
{
	long long a=0,b=0,cnt = 0;
	for (long long i=x1;i<=x2;i++)
	{
		for (long long j=y1;j<=y2;j++)
		{
			if(arr[i][j] == '0')
			{
				a += 1;
			}
			else
			{
				b += 1;
			}
			cnt += 1;
		}
	}
	if (a == b)
	{
		return cnt;	
	}
	else
	{
		return -1;
	}
}

// 0 白色 1 黑色
int main()
{ 
	cin >> n >> m;
	for (long long i=1;i<=n;i++)
	{
		for (long long j=1;j<=m;j++)
		{
			cin >> arr[i][j];
		}
	}
	// 处理数据
	long long x01,y01,x02,y02;
	long long max = -2;
	for (x01=1;x01 <= n;x01++)
	{
		for (y01=1;y01 <= m;y01++)
		{
			for (x02=x01;x02 <= n;x02++)  
			{
				for (y02 = y01;y02 <= m;y02++)
				{
					long long a = func(x01,y01,x02,y02);
					if (a > max)
					{
						max = a;
					}
				}
			}
		}
	}
	cout << max;
	return 0;
}