using System;

​

public class Maths

{

    public int a;

    public int c;

    public int h;

}

​

public class Output

{

    public static void Main(int[] args)

    {

        Maths Maths = new Maths();

        Maths.a = int.Parse(Console.ReadLine());

        Maths.c = int.Parse(Console.ReadLine());

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

        int S = ((Maths.a + Maths.c) * Maths.h) / 2;

        Console.WriteLine("S = {0}",S);

    }

}