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 Java8 (OpenJDK 1.8.0) (judge verdict: Wrong Answer)

import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		// TODO 自動生成されたメソッド・スタブ

		Scanner scan = new Scanner(System.in);

		int a = scan.nextInt();
		int b = scan.nextInt();
		int h = scan.nextInt();

		int total = 0;

		if ((h & 1) == 0) {
			if (a > 0 && b > 0 && h > 0 && a < 100 && b < 100 && h < 100) {
				total = (a + b) * h / 2;
			}
			if (total > 0) {
				System.out.println(total);
			}
		} else {
			System.err.println("");
		}
	}

}