import scala.io.StdIn

object Main {
  def main(args: Array[String]): Unit = {
    val re = """([1-9]{1})\s([1-9]{1})""".r
    StdIn.readLine match {
      case re(a, b) => println(a.toInt * b.toInt)
      case _ => println("-1")
    }
  }
}
