/* package whatever; // don't place package name! */
 
import java.util.*;
import java.lang.*;
import java.io.*;
 
/* Name of the class has to be "Main" only if the class is public. */
class Ideone
{
	public static void main (String[] args) throws java.lang.Exception
	{
		System.out.println(m(new int[] {14,24,34,44}));
		System.out.println(m(new int[] {105,45,-15,-75}));
		System.out.println(m(new int[] {5,25,125,625}));
		System.out.println(m(new int[] {256,64,16,4}));
		System.out.println(m(new int[] {4,2,1,0}));
 
	}
 
	public static int m(int[] a)
    {
        int r = (a[1] > a[0]) ? a[1] / a[0] : a[0] / a[1];
        return (a[0] - a[1] == a[1] - a[2]) ? a[3] - a[0] + a[1] : (a[0] < a[1]) ? a[3] * r : a[3] / r;
    }
 
}

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