using System;

​

public class Program

{

    public static void Main()

    {

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

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

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

        double Parce = (a + b) * h / 3.9;

        Console.WriteLine("трапец = " Parce);

    }

}