{-/⍵{⍵(⍺{+/⍺⍺[⍺{(⍺≥⍵)/⍵}2÷⍨(3×⍵*2)+⍵,-⍵]})¨⍳⍵}≢⍵}
⍺{(⍺≥⍵)/⍵}2÷⍨(3×⍵*2)+⍵,-⍵ ⍝ Basically, get (3×⍵*2 +- ⍵) and check if
⍝ each element is smaller than the length of our list
{-/⍵{⍵(⍺{+/⍺⍺[...]})¨⍳⍵}≢⍵} ⍝ We use these indices to get the elements from the list we're building
⍝ sum up, take the alternating sum, and then return