  // A - 台形 / Trapezoids https://atcoder.jp/contests/abc045/tasks/abc045_a

  import io.Source

  object Main extends App {
    val xs = Source.stdin.getLines()
    val a = xs.next().toInt
    val b = xs.next().toInt
    val h = xs.next().toInt

    println(solve(a, b, h))

    def solve(a: Int, b: Int, h: Int): String = {
      ((a + b) * h / 2).toString
    }
  }
