// By Dhruv Kothari
import java.util.*;
public class p3v2{
    public static void main(String[] args) {
        Scanner scan = new Scanner (System.in);
        int t=scan.nextInt();
        while (t>0) {
           int n=scan.nextInt();
           long a[] = new long  [n+1];
           for (int i=0; i<n; i++)
                a[i]=scan.nextInt();
           long ans=0;
                   if (n==1) {System.out.println(ans); t--; continue;}
                   if (n==2 && a[0]==a[1]) {System.out.println(ans); t--; continue;}
                   if (n==2 && a[0]!=a[1]) {System.out.println("-1"); t--; continue;}

           for (int i=0; i<n-1; i++)
               if (a[i+1]>a[i])  {
                   long dif=a[i+1]-a[i];
                   ans+=2*dif;
                   a[i+1]-=dif;
                   a[i+2]-=dif;
               }
               if (a[n]<0 || a[n-1]<0) ans=-1;
               else {
                   for (int i=n-1; i>1; i--)
                      if (a[i-1]>a[i]) {
                          long dif=a[i-1]-a[i];
                          a[i-2]-=dif;
                          a[i-1]-=dif;
                          ans+=2*dif;
                      }
                      if (a[0]!=a[1] || a[0]!=a[2] || a[0]<0) ans=-1;

               }
for (int i=0; i<n; i++)  if (a[i]<0) ans=-1;


            System.out.println(ans);
           t--;
        }


    }
}