import sys

# sys.setrecursionlimit(200005)
int1 = lambda x: int(x)-1
pDB = lambda *x: print(*x, end="\n", file=sys.stderr)
p2D = lambda x: print(*x, sep="\n", end="\n\n", file=sys.stderr)
def II(): return int(sys.stdin.readline())
def LI(): return list(map(int, sys.stdin.readline().split()))
def LLI(rows_number): return [LI() for _ in range(rows_number)]
def LI1(): return list(map(int1, sys.stdin.readline().split()))
def LLI1(rows_number): return [LI1() for _ in range(rows_number)]
def SI(): return sys.stdin.readline().rstrip()

dij = [(0, 1), (-1, 0), (0, -1), (1, 0)]
# dij = [(0, 1), (-1, 0), (0, -1), (1, 0), (1, 1), (1, -1), (-1, 1), (-1, -1)]
inf = (1 << 63)-1
# md = 10**9+7
md = 998244353

n=II()
aa=LI()

ans=inf
for _ in range(2):
    mxr=[0]*(n+1)
    for i in range(n)[::-1]:
        mxr[i]=max(mxr[i+1],aa[i]+i)
    mxl=0
    for i,a in enumerate(aa[1:],1):
        mxl=max(mxl,aa[i-1]+n-1-i)
        ans=min(ans,max(mxr[i],mxl+n-1-i))
    aa.reverse()

print(ans)