T = int(input())

def func(a, n):
    for i in range(len(a)):
        a[i] += (i+1)*n
    for i in range(N-1):
        if a[i] >= a[i+1]:
            return False
    else:
        return True

for _ in range(T):
    N = int(input())
    A = list(map(int, input().split()))
    a = A[:]
    for i in range(N):
        a[i] += (i+1)*10**12
    for i in range(N-1):
        if a[i] >= a[i+1]:
            print(-1)
            break
    else:
        left = -1
        right = 10**12
        while left+1 < right:
            mid = (left+right)//2
            if not func(A[:], mid):
                left = mid
            else:
                right = mid
        print(right)