list ←  90 47 58 29 22 32 55 5 55 73

      fd   ←  {⍺=0:⍵⋄(⍺-1)∇(1↓⍵)-(¯1↓⍵)}

      1 fd list
¯43 11 ¯29 ¯7 10 23 ¯50 50 18

      2 fd list
54 ¯40 22 17 13 ¯73 100 ¯32