﻿using System;

class Trapezoids
{
    static void Main()
    {
        double a = double.Parse(Console.ReadLine());
        double b = double.Parse(Console.ReadLine());
        double h = double.Parse(Console.ReadLine());
        double area = (a + b) * h / 2;

        Console.WriteLine("Area of Trapezoid is {0}", area);
    }
}

