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 Java (OpenJDK 11.0.6) (judge verdict: Accepted)

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.Arrays;
import java.util.StringTokenizer;

public class Main {
	private static final FastIn in = new FastIn();
	private static final PrintWriter out = new PrintWriter(System.out);

	public static void main(String[] args) {
		var sol = new Solver();
		sol.solve();
		out.close();
	}

	private static final class Solver {
		public void solve() {
			int a = in.nextInt(), b = in.nextInt(), h = in.nextInt();
			out.println((a + b) * h / 2);
		}
	}

	private static final class FastIn {
		private BufferedReader br;
		private StringTokenizer st;

		public FastIn() {
			br = new BufferedReader(new InputStreamReader(System.in), 1 << 20);
			st = null;
		}

		public String next() {
			while (st == null || !st.hasMoreTokens()) {
				try {
					st = new StringTokenizer(br.readLine());
				} catch (IOException e) {
					throw new RuntimeException(e);
				}
			}
			return st.nextToken();
		}

		public int nextInt() {
			return Integer.parseInt(next());
		}
	}
}