05.Trapeziod Area

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace _01.Подготовка
{
    class Program
    {
        static void Main(string[] args)
        {
            double size = double.Parse(Console.ReadLine());
            double size1 = double.Parse(Console.ReadLine());
            double h = double.Parse(Console.ReadLine());
            double area = ((size + size1) * h) / 2.0;
            Console.WriteLine(area);
        }
    }
}