Problem:
Score : 100 points

Problem Statement

You are given a trapezoid. The lengths of its upper base, lower base, and height are a, b, and h, respectively.

An example of a trapezoid

Find the area of this trapezoid.

Constraints

1≦a≦100

1≦b≦100

1≦h≦100

All input values are integers.

h is even.

Input

The input is given from Standard Input in the following format:

a
b
h

Output

Print the area of the given trapezoid. It is guaranteed that the area is an integer.

Sample Input 1

3
4
2

Sample Output 1

7

When the lengths of the upper base, lower base, and height are 3, 4, and 2, respectively, the area of the trapezoid is (3+4)×2/2 = 7.

Sample Input 2

4
4
4

Sample Output 2

16

In this case, a parallelogram is given, which is also a trapezoid.

Solution in C# (Mono 4.6.2.0) (judge verdict: Accepted)

using System;
using System.Collections.Generic;
using System.Linq;

namespace ConsoleApplication
{
    public class Program
    {
        public static void Main(string[] args)
        {
            var a = Utilities.ReadInt();
            var b = Utilities.ReadInt();
            var h = Utilities.ReadInt();
            var result = (a + b) * h / 2;

            System.Console.WriteLine(result);
        }
    }

    public static class Utilities
    {
        #region Extensions for IO
        public static int ReadInt()
            => ReadInts().First();
        public static IEnumerable<int> ReadInts()
            => ReadStrings().Select(int.Parse);

        public static string ReadString()
            => ReadStrings().First();
        public static IEnumerable<String> ReadStrings()
            => Console.ReadLine().Split(' ');
        #endregion

        #region Extensions of int
        public static bool IsOdd(this int n)
            => !n.IsEven();

        public static bool IsEven(this int n)
            => n % 2 == 0;
        
        public static bool CanDivideWith(this int n, int divisor)
            => n % divisor == 0;
        #endregion

        public static int ToInt(this string str)
            => int.Parse(str);
    }
}