using System;

namespace TrapezoidArea
{
    class Program
    {
        static void Main(string[] args)
        {
            double b1 = double.Parse(Console.ReadLine());
            double b2 = double. Parse (Console.ReadLine());
            double h = double.Parse(Console.ReadLine());
            double area = (b1 + b2) * h/2;
            Console.WriteLine(area);
        }
    }
}