IQ Test

给定一个数列的前四个数 $a,b,c,d (1\le a,b,c,d \le 1000)$，如果这是个等差数列或者等比数列且下一项为整数，输出下一项。否则，输出 `42`。

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}+2d $ , $ ... $ , $ 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.

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

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.

836 624 412 200

-12

1 334 667 1000

1333

This problem contains very weak pretests!