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;
using System.Text;
using System.Threading.Tasks;

namespace CS_AtCoder {
	class Program {
		static void Main(string[] args) {
			new Calc().Solve();
		}
	}
	public class Calc {
		public Calc() { }
		public void Solve() {
			int a = Utils.ReadInt();
			int b = Utils.ReadInt();
			int h = Utils.ReadInt();
			((a + b) * h / 2).WL();
			return;
		}
	}
	public static class Utils {
		public static int ReadInt() => int.Parse(Console.ReadLine());
		public static void WL(this object obj) {
			Console.WriteLine(obj);
		}
	}
}