using System;

public class Program

{

    public static void Main(string[] args)

    {

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

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

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

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

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

    }

}