using System;

public class Program

{

    public static void Main()

    { Console.WriteLine("b1=");

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

     Console.WriteLine("b2=");

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

     Console.WriteLine("h=");

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

     var area = (b1 + b2) * h / 2 ;

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

    }

}