let () = Scanf.scanf "%d %d %d" @@ fun n m d ->
  if n - d < 1 then
    print_endline "0.0"
  else if d = 0 then
    Printf.printf "%.10f\n" @@ float_of_int (n - d) /. float_of_int (n * n) *. float_of_int (m - 1)
  else
    Printf.printf "%.10f\n" @@ 2. *. float_of_int (n - d) /. float_of_int (n * n) *. float_of_int (m - 1)
