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: Runtime Error)

using System;
using System.Collections.Generic;
using System.Linq;

namespace AtCoder
{
    class Program
    {
        static void Main()
        {

            var HWN = Utility.ReadLongArray();
            var H = HWN[0];
            var W = HWN[1];
            var N = HWN[2];
            var dic = new Dictionary<long, long>();
            for (var i = 0; i < N; i++)
            {
                var ab = Utility.ReadLongArray();
                var x = ab[0] - 1;
                var y = ab[1] - 1;
                for (long j = 0; j < 3; j++)
                {
                    var currentX = x - j;
                    if (0 <= currentX && currentX < H - 2)
                    {
                        for (long k = 0; k < 3; k++)
                        {
                            var currentY = y - k;
                            if (0 <= currentY && currentY < W - 2)
                            {
                                var key = (currentX << 32) + currentY;
                                if (dic.ContainsKey(key))
                                {
                                    dic[key]++;
                                }
                                else
                                {
                                    dic.Add(key, 1);
                                }
                            }
                        }
                    }
                }

            }
            var ans = new long[10];
            //foreach (var p in dic.GroupBy(v => v.Value))
            //{
            //    ans[p.Key] = p.Count();
            //}
            foreach (var p in dic.Values)
            {
                ans[p]++;
            }
            ans[0] = ((long)H - 2) * (W - 2) - ans.Skip(1).Sum();

            foreach (var an in ans)
            {
                Console.WriteLine(an);
            }
        }
    }

    public static class Utility
    {
        public static string ReadString() => Console.ReadLine();
        public static string[] ReadStringArray() => ReadString().Split(' ');
        public static int ReadInt() => int.Parse(ReadString());
        public static int[] ReadIntArray() => ReadStringArray().ParseInt().ToArray();
        public static long[] ReadLongArray() => ReadStringArray().ParseLong().ToArray();
        public static IEnumerable<int> ParseInt(this IEnumerable<string> source) => source.Select(int.Parse);
        public static IEnumerable<long> ParseLong(this IEnumerable<string> source) => source.Select(long.Parse);
    }
}