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
{
    // http://abc045.contest.atcoder.jp/tasks/abc045_a
    public class Abc045_a
    {
#if !Development
		public static void Main(string[] args)
		{
			Abc045_a.Solve(args);
		}
#endif

        public static void Solve(string[] args)
        {
            var a = ReadLineInt()[0];
            var b = ReadLineInt()[0];
            var h = ReadLineInt()[0];

            var result =((a + b) * h) / 2;


            Console.WriteLine(result.ToString());
        }

        public static List<string> ReadLineString() { return Console.ReadLine().Split(' ').ToList(); }
        public static List<int> ReadLineInt() { return Console.ReadLine().Split(' ').Select(str => int.Parse(str)).ToList(); }
    }
}