using System;

​

public class Program

{

    public static void Main()

    {

        var b1 = double.Parse(Console.ReadLine());

        var b2 = double.Parse(Console.ReadLine());

        var h = double.Parse(Console.ReadLine());

        //

        var result = (b1+b2)*h/2;

        Console.WriteLine(result);

    }

}