Jolf, 13 bytes

ΆFi liγhj_γgi

A tranpilation of the JavaScript answer.

Explanation:

ΆFi liγhj_γgi
Ά             ternary add
 Fi            i[0],
   `li         i sliced
      γhj       γ = j + 1
         _γ     to -γ
           gi  and the last of i

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