using System;

​

​

public class Program

{

    public void Trapec(){

int b1 = int.Parse(Console.ReadLine());

int b2 = int.Parse(Console.ReadLine());

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

Console.WriteLine((b1+b2)*h/2); 

​

}

    public static void Main()

    {

        Program program= new Program();

        program.Trapec();

    }

}