from collections import*
for _ in range(int(input())):
  n=int(input())
  *a,=map(int,input().split())
  d=defaultdict(int)
  for i in range(n-1):
    j=i+1
    ni,nj=i+1,j+1
    if a[i]<a[j]:
      d[0]+=1
      continue
#    a[i]+ni*m<a[j]+nj*m
#    a[i]-a[j]<m*(nj-ni)
    m=(a[i]-a[j])//(nj-ni)+1
    d[m]+=1
#  print(d)
  s=0
  ans=-1
  for k in sorted(d):
    s+=d[k]
    if s>=n-1:
      ans=k
      break
  print(ans)