use std::io::{self, Stdin};
use std::str::{self, FromStr};
use std::error::Error;
use std::thread;
use std::cmp::*;
const INF: i64 = 1e16 as i64;
fn exec() {
    let mut sc = Scanner::new();
    let n: usize = sc.ne();
    let a: Vec<i64> = (0..n).map(|_| sc.ne()).collect();
    let mut dp_l = vec![vec![INF; n]; n];
    let mut dp_r = dp_l.clone();
    dp_l[0][n - 1] = a[0];
    dp_r[0][n - 1] = a[n - 1];
    for len in (1..n).rev() {
        for i in 0..n - len + 1 {
            let j = i + len - 1;
            let mut res_l = INF;
            let mut res_r = INF;
            if i > 0 {
                res_l = min(res_l, dp_l[i - 1][j] + 1);
                res_r = min(res_r, dp_l[i - 1][j] + len as i64);
            }
            if j < n - 1 {
                res_l = min(res_l, dp_r[i][j + 1] + len as i64);
                res_r = min(res_r, dp_r[i][j + 1] + 1);
            }
            dp_l[i][j] = max(a[i], res_l);
            dp_r[i][j] = max(a[j], res_r);
        }
    }
    let ans = (0..n).fold(INF, |mi, i| min(mi, dp_l[i][i]));
    println!("{}", ans);
}

fn main() {
    const DEFAULT_STACK: usize = 16 * 1024 * 1024;
    let builder = thread::Builder::new();
    let th = builder.stack_size(DEFAULT_STACK);
    let handle = th.spawn(|| { exec(); }).unwrap();
    let _ = handle.join();
}

#[allow(dead_code)]
struct Scanner {
    stdin: Stdin,
    id: usize,
    buf: Vec<u8>,
}

#[allow(dead_code)]
impl Scanner {
    fn new() -> Scanner {
        Scanner {
            stdin: io::stdin(),
            id: 0,
            buf: Vec::new(),
        }
    }
    fn next_line(&mut self) -> Option<String> {
        let mut res = String::new();
        match self.stdin.read_line(&mut res) {
            Ok(0) => return None,
            Ok(_) => Some(res),
            Err(why) => panic!("error in read_line: {}", why.description()),
        }
    }
    fn next<T: FromStr>(&mut self) -> Option<T> {
        while self.buf.len() == 0 {
            self.buf = match self.next_line() {
                Some(r) => {
                    self.id = 0;
                    r.trim().as_bytes().to_owned()
                }
                None => return None,
            };
        }
        let l = self.id;
        assert!(self.buf[l] != b' ');
        let n = self.buf.len();
        let mut r = l;
        while r < n && self.buf[r] != b' ' {
            r += 1;
        }
        let res = match str::from_utf8(&self.buf[l..r]).ok().unwrap().parse::<T>() {
            Ok(s) => Some(s),
            Err(_) => panic!("parse error"),
        };
        while r < n && self.buf[r] == b' ' {
            r += 1;
        }
        if r == n {
            self.buf.clear();
        } else {
            self.id = r;
        }
        res
    }
    fn ne<T: FromStr>(&mut self) -> T {
        self.next::<T>().unwrap()
    }
}