import java.io.*;
import java.util.*;
public class Arrr{
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int t = sc.nextInt();

    while(t-- >0){

      int n = sc.nextInt();
      int a[] = new int[n];

      for(int i = 0; i <n; i++){
        a[i] = sc.nextInt();
      }


      int sum = 0;
      for(int i = 0; i <n-1; i++){

        sum += Math.abs(a[i]-a[i+1]);
      }

      if(sum%2!=0){
        System.out.println(sum);
      }
      else{
        System.out.println("-1");
      }
    }
  }
}