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 area = (b1 + b2) * h / 2;

        Console.WriteLine(area);

    }

}

​