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 static System.Console;
using static System.Math;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace AtCoder
{
    public class IO
    {
        public static string ReadStr() => Console.ReadLine();
        public static string[] ReadStrArr() => ReadStr().Split(' ');
        public static int ReadInt() => int.Parse(ReadStr());
        public static int[] ReadIntArr() => Array.ConvertAll(Console.ReadLine().Split(' '), int.Parse);
        public static double ReadDouble() => double.Parse(ReadStr());
        public static double[] ReadDoubleArr() => Array.ConvertAll(Console.ReadLine().Split(' '), double.Parse);
        public static long ReadLong() => long.Parse(ReadStr());
        public static long[] ReadLongArr() => Array.ConvertAll(Console.ReadLine().Split(' '), long.Parse);
    }

    class Template
    {
        static void Main(string[] args)
        {
            int a = IO.ReadInt();
            int b = IO.ReadInt();
            int h = IO.ReadInt();
            WriteLine((a + b) * h / 2);
        }
    }
}