let rec takeFirstN (xs : string list) (n : int) (acc : string) : string =
    match n with
    | 0 -> acc
    | x -> takeFirstN (List.tl xs) (n-1) (acc ^ (List.nth xs 0));;

let rec joinK (xs : string list) (acc : string list) (k : int) : string list =
  if List.length xs < k then acc
  else joinK (List.tl xs) (acc @ [takeFirstN xs k ""]) k;;

let longestConsec (xs : string list) (k : int) : string =
  if (List.length xs) == 0 || (List.length xs) < k || k <= 0
  then ""
  else
    let pairs = joinK xs [] k in
    List.fold_left (fun acc i -> if (String.length i) > (String.length acc) then i else acc) "" pairs;;