import java.io.FileInputStream;
import java.io.IOException;
import java.util.Scanner;
 
class QBIT04
{
    public static void main(String[] args) throws IOException
    {
        Scanner sc=new Scanner(System.in);
        int tc=sc.nextInt();
        while(tc-->0)
        {
            int n=sc.nextInt();
            long sum=0;
            for(int i=0;i<n;i++)
                sum+=sc.nextInt();
            if(sum!=0 && sum%n==0)
                System.out.println((sum/n));
            else
                System.out.println("-1");
        }
    }
}

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MgozCjAgMiA0CjQKMiA0IDEgMg==

2
3
0 2 4
4
2 4 1 2