Q=int(input())
for _ in range(Q):
  N=int(input())
  A=list(map(int,input().split()))
  l=0
  for i in range(1,N):
    y=A[i-1]-A[i]+1
    if y>0:
      l=max(l,y)
  B=[0]*N
  for i in range(N):
    B[i]=A[i]+l*(i+1)
  ans=True
  for i in range(1,N):
    if B[i]<=B[i-1]:
      ans=False
      break
  if ans==True:
    print(l)
  else:
    print(-1)