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

namespace trapezoid_area
{
    class Program
    {
        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) * 2 / 2.0;
            Console.WriteLine("trapezoid area = " + area);
        }
    }
}