let () =
  Scanf.scanf "%d %d\n" @@ fun a b ->
  if abs (a-b) mod 2 = 0 then 
    if a > b then (print_int (a-((a-b)/2)); print_newline ())
    else (print_int (b-((b-a)/2)); print_newline ())
  else print_endline "IMPOSSIBLE"