3493 -- Chessboard Puzzle

Language:

Chessboard Puzzle

Time Limit: 2000MS		Memory Limit: 131072K
Total Submissions: 588		Accepted: 280

Description

Farmer John has invented a game, Chessboard Puzzle, which perplexes lots of puzzle fanatics. The game is quite simple. Given a chessboard in such a way that every grid is presented by a specified integer, you are obliged to color every grid in white or black. For each pair of grids subjected to following restraints:

they are neighbor, in detail, sharing common edge;
they are colored differently.

add the product of the two specified integers to the score. And your goal is to maximum the total score. Could you write a program to tell John the maximum score?

Input

The input describes one puzzle which begins with two integer n, m(1 ≤ n, m ≤16). Then n lines follow. Each contains m integers. The j-th number on i-th row corresponds to the grid(i,j). The absolute value of each number is less than 1000.

Output

Print the maximum score in a line. 　

Sample Input

5 5
-136 -712 -286 -622 564 
6 -899 623 -152 -519 
904 982 593 799 93 
-815 122 -307 572 -359 
921 757 -888 351 935

Sample Output

Source

POJ Founder Monthly Contest – 2008.01.31, Qi-feng CHEN