using System;

public class Program

{

    static double f(double a, double b, double h)

    {

        double s = (a+b)*h/2;

        return s;

    }

    public static void Main()

    {

        Console.WriteLine("Hello World");

        double a = double.Parse(Console.ReadLine());

        double b = double.Parse(Console.ReadLine());

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

        Console.WriteLine(f(a, b , h));

    }

}