object Main {

  import java.util.Scanner

  def main(args: Array[String]): Unit = {
    val scanner = new Scanner(System.in)
    var str = scanner.next()
    var ref = 0
    val a = str.toInt
    str = scanner.next()
    val b = str.toInt
    str = scanner.next()
    val h = str.toInt

    ref = (a + b) * h / 2

    println(ref)

  }
}
