object Main extends App {
	val KAB = scala.io.StdIn.readLine.split(" ").map(_.toLong)

	val sum: Long = KAB(1) - KAB(2)

	if(sum <=0){
		if(KAB(0) <= KAB(1)) println(1)
		else println(-1)
	}else{
		val k1 = KAB(0) - KAB(1)
		val a = k1/sum

		if(a*sum >= k1) println(2*a + 1)
		else println(2*(a+1)+1)
	}
}