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 AtCoder
{
    class Program
    {
        static void Main(string[] args)
        {
            //[summary]A - 台形 / Trapezoids
            var lines = ReadLines(3);
            int a = lines[0];
            int b = lines[1];
            int h = lines[2];

            Console.WriteLine((a + b) * h / 2);
        }

        static List<int> ReadLines(int count = 0)
        {
            //[summary]コンソールの値を読み込む
            var lines = new List<int>();
            string s;

            //読み込む行数が未指定のときは、1行目の値を"count"とみなす
            if (count == 0)
            {
                s = Console.ReadLine().TrimEnd();
                count = int.Parse(s);
            }

            for (int i = 0; i < count; i++)
            {
                s = Console.ReadLine().TrimEnd();
                lines.Add(int.Parse(s));
            }

            return lines;
        }
    }
}