let () =
  Scanf.scanf "%d %d %s" @@ fun n k s ->
  let s = Array.init n (fun i -> s.[i]) in
  s.(k-1) <- (match s.(k-1) with 'A' -> 'a' | 'B' -> 'b' | _ -> 'c');
  Array.iter print_char s;
  print_newline ()