import sys

sys.setrecursionlimit(int(1e6))
input = lambda: sys.stdin.readline().rstrip("\r\n")
MOD = 998244353
INF = int(4e18)


def min2(a: int, b: int) -> int:
    return a if a < b else b


def max2(a: int, b: int) -> int:
    return a if a > b else b


if __name__ == "__main__":
    n = int(input())
    nums = list(map(int, input().split()))

    dp = [[INF] * n for _ in range(n)]
    dp[0][n - 1] = 0
    dp[n - 1][0] = 0
    for w in range(n, 1, -1):
        for i in range(n - w + 1):
            j = i + w - 1
            dp[i][j] = min2(dp[i][j], dp[j][i] + (j - i - 1))
            dp[j][i] = min2(dp[j][i], dp[i][j] + (j - i - 1))
            dp[i + 1][j] = min2(dp[i + 1][j], max2(dp[i][j], nums[i]) + 1)
            dp[j - 1][i] = min2(dp[j - 1][i], max2(dp[j][i], nums[j]) + 1)

    res = INF
    for i in range(n):
        res = min2(res, max2(dp[i][i], nums[i]))
    print(res)