/* 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 int twiceIndex(int[] arr) { 
 
		int maxIndex = 0; 
 
		for (int i = 0; i < arr.length; ++i) { 
			if (arr[i] > arr[maxIndex]) 
				maxIndex = i; 
		} 
		for (int i = 0; i < arr.length; ++i) { 
			if (maxIndex != i && arr[maxIndex] < 2 * arr[i]) 
			return -1; 
		} 
		return maxIndex; 
	}
	public static void main (String[] args) throws java.lang.Exception
	{
		// your code goes here
		int[] arr = {3, 6, 0, 1};
 
		System.out.println(twiceIndex(arr));
	}
}

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