(* Enter your code here. Read input from STDIN. Print output to STDOUT *)
let rec read_lines acc = try
    let line = input_line stdin |> int_of_string in
    read_lines (line :: acc)
  with End_of_file -> List.rev acc;;

let arr = read_lines [];;

(* This commented code below increased space complexity in my previous submission
let len = List.length arr;;

let rec range start stop step acc =
  if start >= stop then
    List.rev acc
  else
    range (start + step) stop step (start :: acc);;

let indices = range 0 len 1 [];;

let pairs = List.combine indices arr;;*)

(* Better code is here below *)
let enumerate lst =
  List.mapi (fun index elem -> (index, elem)) lst;;

let pairs = enumerate arr;;

let result = List.filter (fun (index,el) -> index mod 2 <> 0) pairs;;

let () = 
  List.iter (fun (index, el) -> print_endline (string_of_int el)) result;;