using System;

public class Program

{

    public static void Main()

    {

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

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

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

        double area= (b1+b2)*h/2.0;

        Console.WriteLine("Trapezoid area= "+ area);

    }

}