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

namespace _05_TrapeziodArea
{
    class TrapeziodArea
    {
        static void Main(string[] args)
        {
            float b1 = float.Parse(Console.ReadLine());
            float b2 = float.Parse(Console.ReadLine());
            float h = float.Parse(Console.ReadLine());
            float area = (b1 + b2) * h / 2;
            Console.WriteLine("Trapezoid area = " + area);
        }
    }
}