Problem:
Score : 100 points

Problem Statement

There are H rows and W columns of white square cells.

You will choose h of the rows and w of the columns, and paint all of the cells contained in those rows or columns.

How many white cells will remain?

It can be proved that this count does not depend on what rows and columns are chosen.

Constraints

All values in input are integers.

1 \leq H, W \leq 20

1 \leq h \leq H

1 \leq w \leq W

Input

Input is given from Standard Input in the following format:

H W
h w

Output

Print the number of white cells that will remain.

Sample Input 1

3 2
2 1

Sample Output 1

1

There are 3 rows and 2 columns of cells. When two rows and one column are chosen and painted in black, there is always one white cell that remains.

Sample Input 2

5 5
2 3

Sample Output 2

6

Sample Input 3

2 4
2 4

Sample Output 3

0

Solution in C# (Mono 4.6.2.0) (judge verdict: Accepted)

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Linq;
using System.Text;
using static System.Console;

namespace ABC121A
{
    static class Program
    {
        static void Main(string[] args)
        {
            var line1 = ReadLine().SplitTryParseToList<int>();
            var line2 = ReadLine().SplitTryParseToList<int>();

            WriteLine((line1[0] - line2[0]) * (line1[1] - line2[1]));
        }

        public static T TryParse<T>(this String input)
        {
            try
            {
                var converter = TypeDescriptor.GetConverter(typeof(T));
                if (converter != null)
                {
                    return (T)converter.ConvertFromString(input);
                }
                else
                {
                    throw new InvalidCastException("");
                }
            }
            catch
            {
                throw new InvalidCastException(typeof(T) + " is not supported.");
            }
        }

        public static List<T> SplitTryParseToList<T>(this String input)
        {
            return input.Split().Select(n => n.TryParse<T>()).ToList();
        }

        public static List<T> ListSwap<T>(this List<T> list, Int32 index1, Int32 index2)
        {
            var t = list[index1];
            list[index1] = list[index2];
            list[index2] = t;
            return list;
        }

        public static List<Tuple<T, int>> DuplicateCount<T>(this IEnumerable<T> list)
        {
            return list
                .GroupBy(i => i)
                .Where(g => g.Any())
                .Select(g => Tuple.Create(g.Key, g.Count()))
                .ToList();
        }
        public static List<Tuple<T, int>> DuplicateSort<T>(this IEnumerable<Tuple<T, int>> list)
        {
            return list.OrderByDescending((x) => x.Item2).ToList();
        }
        public static List<T> ReadLineOne<T>(int n)
        {
            var list = new List<T>();
            foreach (var i in Enumerable.Range(1, n))
            {
                list.Add(Console.ReadLine().TryParse<T>());
            }
            return list;
        }
        public static void PrintAll<T>(this IEnumerable<T> list)
        {
            foreach (var i in list)
            {
                Console.Write($"{i} ");
            }

            Console.WriteLine();
        }
    }
}