j^ i>=2          
  | | j<=n+2-i    
  | |\     | i<=n
  | | \    |     
m-+-****---+-j<=m
  | *****  |     
  | ****** |     
  | *******|     
2-+-********-j>=2
  | |      |\    
0-+-+------+--->i
  | |      |     
  0 2      n