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.Linq;

class Program {
    static void Main(string[] args) {
        int a = cin.ni();
        int b = cin.ni();
        int h = cin.ni();

        Console.WriteLine((a + b) * h / 2);

    }
}


public class cin {
    private static string[] s = new string[0];
    private static int i = 0;

    private static char[] cs = new char[] { ' ' };

    public static string next() {
        if (i < s.Length) return s[i++];
        string st = Console.ReadLine();
        while (st == "") st = Console.ReadLine();
        s = st.Split(cs, StringSplitOptions.RemoveEmptyEntries);
        i = 0;
        return next();
    }

    public static int ni() {
        return int.Parse(next());
    }

    public static long nextLong() {
        return long.Parse(next());
    }

    public static double nextDouble() {
        return double.Parse(next());
    }

}