#!/usr/bin/ python3.8
import sys
read = sys.stdin.buffer.read
readline = sys.stdin.buffer.readline
readlines = sys.stdin.buffer.readlines

N, *A = map(int, read().split())

assert N == len(A)

def calc_dp(A):
    """時刻 t に左端を出発して全部拾っていった場合に、そのセルのものを拾う時刻。max(t + i, x)"""
    dp = [-1] * N
    for i, x in enumerate(A):
        dp[i] = max(dp[i - 1] + 1, x)
    return dp


dp1 = calc_dp(A)
dp2 = calc_dp(A[::-1])[::-1]

def gen_cand():
    global dp1, dp2
    for _ in range(2):
        dp1, dp2 = dp2[::-1], dp1[::-1]
        yield dp1[-1]
        for i, (x, y) in enumerate(zip(dp1, dp2[1:])):
            yield max(y, x + 2 * (N - i) - 3)


answer = min(gen_cand())
print(answer)