/* 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
	{
		 int[] arr = {-1,2,-3,3};
    	System.out.println(findMax(arr));
 	}
	  
	 static int findMax(int[] nums) {
     
     int max = Integer.MIN_VALUE; 


     int n = nums.length;
     HashSet<Integer> set  = new HashSet<>();

     for(int i=0;i<n;i++){
      set.add(nums[i]);
     }

     for(int i=0;i<n;i++){
        if(set.contains(-nums[i])){
            max = Math.max(max,Math.abs(nums[i]));
        }
     }
     if(max == Integer.MIN_VALUE){
         return -1;
     }
     
     return max;
     
    }
}