=4
(1 0 0 0
 0 1 0 0
 0 0 1 0
 0 0 0 1)
  =5
(1 0 0 0 0
 0 1 0 0 0
 0 0 1 0 0
 0 0 0 1 0
 0 0 0 0 1)