7
 0  20  0  0  0 23  1	
20   0 15  0  0  0  4
 0  15  0  3  0  0  9
 0   0  3  0 17  0 16
 0   0  0 17  0 28 25
23   0  0  0 28  0 36  
 1   4  9 16 25 36  0 