type [<Measure>] s = static member toS (x: int64) = x * 1L<s>
type [<Measure>] m =
  static member m2s (m: int64<m>) = m * 60L<s/m>
  static member toM (x: int64) = x * 1L<m>
type [<Measure>] h =
  static member h2s (h: int64<h>) = h * 3600L<s/h>
  static member toH (x: int64) = x * 1L<h>

let solve a b c =
  let a' = a |> m.m2s
  let c' = c |> h.h2s
  let x = (c', a' - b)
  match x with
  | _, x when x < 0L<s> -> None
  | x, y when x % y = 0L<s> -> Some(x / y)
  | x, y -> Some(x / y + 1L)

let A, B, C =
  let t = stdin.ReadLine().Split() |> Array.map int64
  m.toM t.[0], s.toS t.[1], h.toH t.[2]

solve A B C
|> function | Some(x) -> x | None -> -1L
|> stdout.WriteLine