n=int(input())
a=list(map(int,input().split()))
s=sum(a)
ans=[]
for i in range(1,1000):
  l=[]
  for j in range(i):
    l.append(j+1)
  for j in range(i-2,-1,-1):
    l.append(j+1)
  if sum(l)>s:
    break
  else:
    ct=0
    ctb=0
    for j in range(min(len(l),len(a))):
      if a[j]>l[j]:
        ct+=a[j]-l[j]
    if len(l)<len(a):
      for j in range(len(l),len(a)):
        ct+=a[j]
    ans.append(ct)
print(min(ans))