import strutils


proc nextString: string =
   result = ""
   while not endOfFile stdin:
      let nextChar = readChar stdin
      case nextChar
      of '\r':
         discard
      of "\n"[0], ' ':
         break
      else:
         add result, nextChar


proc nextInt: int =
   return parseInt nextString()


proc main: void =
   let hand = nextInt() * 60
   let runCost = nextInt()
   let initCost = nextInt() * 3600

   if hand <= runCost:
      echo -1
      return

   echo (initCost - 1) div (hand - runCost) + 1


when isMainModule:
   main()