flax, 67 bytes
⊃ḟκ→∴€∵µḟκ→A∴
Attempt This Online!Attempt This Online!
Explained
⊃ḟκ→∴€∵µḟκ→A∴
⊃ ⊳∵ head of
€ ⊳ last four things as amin monad:of
κ→∴ κ→A∴ ⊳ range(1, abs(max(input)) + 12)
ḟ ⊳ set differencediff with input