let sum_second_col text =
  let lines = String.split_on_char '\n' text in
  List.fold_left (fun acc line ->
    let fields = String.split_on_char ',' line in
    if List.length fields >= 2 then
      acc + int_of_string (List.nth fields 1)
    else acc
  ) 0 lines

;;

sum_second_col "a,1,extra\nb,2,extra\nc,3,extra\nd,4,extra"