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;

public class Input
{
    public long a;

    public long b;

    public long h;

    public Input()
    {
        string[] input = null;

        // スペース区切りの整数の入力(横、個別)
        input = Console.ReadLine().Split(' ');

        a = long.Parse(input[0]);

        // スペース区切りの整数の入力(横、個別)
        input = Console.ReadLine().Split(' ');

        b = long.Parse(input[0]);

        // スペース区切りの整数の入力(横、個別)
        input = Console.ReadLine().Split(' ');

        h = long.Parse(input[0]);
    }
}

public class Program
{
    public static void Main(string[] args)
    {
        string output = Proc();

        // 出力
        Console.WriteLine(output);
    }

    static string Proc()
    {
        Input input = new Input();
        string output = string.Empty;

        long result = -1;

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

        output = result.ToString();
        return output;
    }
}