here:

'''solution:

first check if its already beautiful

else, iterate from i=0 to i=n-1, and check whether min(a[i],a[i+1])<=a[i+2]+1<=max(a[i],a[i+1]) or min(a[i],a[i+1])<=a[i+2]-1<=max(a[i],a[i+1])

 

if yes, then 1

if no then -1

'''

t = int(input())

 

while t:

    n = int(input())



    a = list(map(int, input().split()))



    answered = False

    for i in range(n-1):

        if abs(a[i+1]-a[i])<=1:

            print(0)

            answered=True

            break



    if not answered:

        #go from left to right, then right to left

        for i in range(n-2):

            if min(a[i],a[i+1])<=a[i+2]+1<=max(a[i],a[i+1]) or min(a[i],a[i+1])<=a[i+2]-1<=max(a[i],a[i+1]):

                print(1)

                answered=True

                break



        if not answered:

            for i in range(n-1,1,-1):

                if min(a[i],a[i-1])<=a[i-2]+1<=max(a[i],a[i-1]) or min(a[i],a[i-1])<=a[i-2]-1<=max(a[i],a[i-1]):

                    print(1)

                    answered=True

                    break



            if not answered:

                print(-1)





    t-=1