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

N = int(readline())
A = np.array(read().split(), np.int64)
INF = 10 ** 9 * 2

n = 1
dpl = np.array([A[0]], np.int64)
dpr = np.array([A[-1]], np.int64)

for n in range(2, N + 1):
    newdpl = np.full(n, INF, np.int64)
    np.minimum(newdpl[1:], dpl + 1, out=newdpl[1:])
    np.minimum(newdpl[:-1], dpr[::-1] + N - n + 1, out=newdpl[:-1])
    np.maximum(newdpl, A[:n], out=newdpl)

    newdpr = np.full(n, INF, np.int64)
    np.minimum(newdpr[1:], dpr + 1, out=newdpr[1:])
    np.minimum(newdpr[:-1], dpl[::-1] + N - n + 1, out=newdpr[:-1])
    np.maximum(newdpr, A[::-1][:n], out=newdpr)
    dpl, dpr = newdpl, newdpr

answer = np.min(dpl)
print(answer)