object Main extends App {

  import scala.io.StdIn._

  val Array(a, b, c) = readLine.trim.split(' ').map(_.toInt)
  println(a * b / 2)
  implicit class Extension[T](val value :T) extends AnyVal {
    @inline
    def let[A](@inline func: T ⇒ A): A = func(value)
    @inline
    def also(@inline func: T ⇒ Unit): T = {func(value); value}
  }
}