Problem:
Score : 300 points

Problem Statement

Takahashi, Nakahashi and Hikuhashi have integers A, B and C, respectively.
After repeating the following operation K times, find the integer Takahashi will get minus the integer Nakahashi will get:

Each of them simultaneously calculate the sum of the integers that the other two people have, then replace his own integer with the result.

However, if the absolute value of the answer exceeds 10^{18}, print Unfair instead.

Constraints

1 \leq A,B,C \leq 10^9

0 \leq K \leq 10^{18}

All values in input are integers.

Input

Input is given from Standard Input in the following format:

A B C K

Output

Print the integer Takahashi will get minus the integer Nakahashi will get, after repeating the following operation K times.
If the absolute value of the answer exceeds 10^{18}, print Unfair instead.

Sample Input 1

1 2 3 1

Sample Output 1

1

After one operation, Takahashi, Nakahashi and Hikuhashi have 5, 4 and 3, respectively. We should print 5-4=1.

Sample Input 2

2 3 2 0

Sample Output 2

-1

Sample Input 3

1000000000 1000000000 1000000000 1000000000000000000

Sample Output 3

0

Solution in C# (Mono 4.6.2.0) (judge verdict: Time Limit Exceeded)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Numerics;


namespace practice1
{
    class Program
    {
        static void Main(string[] args)
        {
            BigInteger[] buff = MYCP.ReadBigIntegers(4);
            BigInteger a, b, c, k, x, na, nb, nc;
            a = buff[0];
            b = buff[1];
            c = buff[2];
            k = buff[3];

            for(int i = 0; i < k; i++)
            {
                na = b + c;
                nb = a + c;
                nc = a + b;
                a = na;
                b = nb;
                c = nc;
            }

            x = a - b;

            a = 1;
            for (int i = 0; i < 18; i++)
            {
                a *= 10;
            }

            b = x;
            if (b < 0) b *= -1;

            if (b > a) MYCP.Println("Unfair");
            Console.WriteLine(x);

            

        }




    }

    class MYCP
    {
        //標準出力の後改行する
        public static void Println(string str)
        {
            Console.WriteLine(str);
        }

        //標準出力の後改行しない
        public static void Print(string str)
        {
            Console.Write(str);
        }


        //int変数を標準出力する
        public static void PrintNumber(long number)
        {
            Console.WriteLine("" + number);
        }

        //int変数の配列を標準出力する
        public static void PrintNumbers(int[] a)
        {
            if (a.Length == 0) return;

            Console.Write(a[0]);

            for(int i = 1; i < a.Length; i++)
            {
                Console.Write(" ");
                Console.Write(a[i]);
            }

            Console.WriteLine();
        }


        //文字列を配列化して返す
        public static string[] MyReadLineSplit()
        {
            return Console.ReadLine().Split();
        }


        //double変数をソートする
        public static void MyDoubleSort(double[] boo)
        {
            if (boo.Length < 2) return;

            long i, count;
            double a;

            while (true)
            {
                count = 0;

                for (i = 0; i < boo.Length - 1; i++)
                {
                    if (boo[i] > boo[i + 1])
                    {
                        a = boo[i];
                        boo[i] = boo[i + 1];
                        boo[i + 1] = a;
                        count++;
                    }
                }

                if (count == 0) break;
            }
        }

        //int変数をソートする
        public static void MyIntSort(int[] boo)
        {
            if (boo.Length < 2) return;

            long i, count;
            int a;

            while (true)
            {
                count = 0;

                for (i = 0; i < boo.Length - 1; i++)
                {
                    if (boo[i] > boo[i + 1])
                    {
                        a = boo[i];
                        boo[i] = boo[i + 1];
                        boo[i + 1] = a;
                        count++;
                    }
                }

                if (count == 0) break;
            }
        }

        //指定された個数分int変数を読み取る
        public static int[] ReadNumbers(long number)
        {
            int[] a = new int[number];
            string[] buff = MyReadLineSplit();
            for (long i = 0; i < number; i++)
            {
                a[i] = int.Parse(buff[i]);
            }

            return a;
        }

        //指定された個数分BigInteger変数を読み取る
        public static BigInteger[] ReadBigIntegers(long number)
        {
            BigInteger[] a = new BigInteger[number];
            string[] buff = MyReadLineSplit();
            for(long i = 0; i < number; i++)
            {
                a[i] = BigInteger.Parse(buff[i]);
            }

            return a;
        }



        //渡された自然数が素数か判定する。素数ならtrueを返す。
        public static bool PrimeCheck(int number)
        {
            if (number < 2) return false;

            for(int i = 2; i < number; i++)
            {
                if (number % i == 0) return false;
            }

            return true;
        }

        //渡された自然数が累乗数か判定する。累乗数ならtrueを返す。
        public static bool Bekizyousuu_Check(int number)
        {
            if (number == 1) return true;

            int i, copy;


            for (i = 2; i < number; i++)
            {
                copy = number;
                while (copy % i == 0)
                {
                    copy /= i;
                }

                if (copy == 1) return true;
            }

            return false;
        }
    }
}