CF328A.IQ Test

Petya is preparing for IQ test and he has noticed that there many problems like: you are given a sequence, find the next number. Now Petya can solve only problems with arithmetic or geometric progressions.

Arithmetic progression is a sequence _a_1, _a_1 + d, a_1 + 2_d, ..., _a_1 + (n - 1)d, where _a_1 and d are any numbers.

Geometric progression is a sequence _b_1, b_2 = b_1_q, ..., b__n = b__n - 1_q, where _b_1 ≠ 0, q ≠ 0, q ≠ 1.

Help Petya and write a program to determine if the given sequence is arithmetic or geometric. Also it should found the next number. If the sequence is neither arithmetic nor geometric, print 42 (he thinks it is impossible to find better answer). You should also print 42 if the next element of progression is not integer. So answer is always integer.

佩佳正在为智商测试做准备，他注意到有很多这类题目：给你一个数列，求它的下一个数字。目前佩佳只能解决等差数列或等比数列的问题。

等差数列是一个形如 $a_1,\ a_1 + d,\ a_1 + 2d,\ \dots,\ a_1 + (n - 1)d$ 的序列，其中 $a_1$ 和 $d$ 可以是任意实数。

等比数列是一个形如 $b_1,\ b_2 = b_1 q,\ \dots,\ b_n = b_{n-1} q$ 的序列，其中 $b_1 \neq 0$，$q \neq 0$，且 $q \neq 1$。

请帮助佩佳编写一个程序，判断给定的数列是等差数列还是等比数列，并求出它的下一个数字。如果该数列既不是等差数列也不是等比数列，则输出 42（他认为这是无法找到更好答案的情况）。此外，如果该数列所对应的等差或等比数列的下一个元素不是整数，也应输出 42。因此，最终答案始终为整数。

The first line contains exactly four integer numbers between 1 and 1000, inclusively.

第一行恰好包含四个介于 1 到 1000（含）之间的整数。

Print the required number. If the given sequence is arithmetic progression, print the next progression element. Similarly, if the given sequence is geometric progression, print the next progression element.

Print 42 if the given sequence is not an arithmetic or geometric progression.

输出所需的数字。如果给定序列是等差数列，则输出该等差数列的下一个项；如果给定序列是等比数列，则输出该等比数列的下一个项。

如果给定序列既不是等差数列也不是等比数列，则输出 42。

This problem contains very weak pretests!

本题的预测试非常弱！