#[allow(unused_imports)]
use std::cmp::*;
#[allow(unused_imports)]
use std::collections::*;
use std::io::{Write, BufWriter};
// https://qiita.com/tanakh/items/0ba42c7ca36cd29d0ac8
macro_rules! input {
    (source = $s:expr, $($r:tt)*) => {
        let mut iter = $s.split_whitespace();
        let mut next = || { iter.next().unwrap() };
        input_inner!{next, $($r)*}
    };
    ($($r:tt)*) => {
        let stdin = std::io::stdin();
        let mut bytes = std::io::Read::bytes(std::io::BufReader::new(stdin.lock()));
        let mut next = move || -> String{
            bytes
                .by_ref()
                .map(|r|r.unwrap() as char)
                .skip_while(|c|c.is_whitespace())
                .take_while(|c|!c.is_whitespace())
                .collect()
        };
        input_inner!{next, $($r)*}
    };
}

macro_rules! input_inner {
    ($next:expr) => {};
    ($next:expr, ) => {};

    ($next:expr, $var:ident : $t:tt $($r:tt)*) => {
        let $var = read_value!($next, $t);
        input_inner!{$next $($r)*}
    };
}

macro_rules! read_value {
    ($next:expr, ( $($t:tt),* )) => {
        ( $(read_value!($next, $t)),* )
    };

    ($next:expr, [ $t:tt ; $len:expr ]) => {
        (0..$len).map(|_| read_value!($next, $t)).collect::<Vec<_>>()
    };

    ($next:expr, chars) => {
        read_value!($next, String).chars().collect::<Vec<char>>()
    };

    ($next:expr, usize1) => {
        read_value!($next, usize) - 1
    };

    ($next:expr, [ $t:tt ]) => {{
        let len = read_value!($next, usize);
        (0..len).map(|_| read_value!($next, $t)).collect::<Vec<_>>()
    }};

    ($next:expr, $t:ty) => {
        $next().parse::<$t>().expect("Parse error")
    };
}

fn ok(a: &[i64], k: usize) -> bool {
    let mut cur: Vec<(_, usize)> = Vec::new();
    let mut curlen = 0;
    for &a in a {
        if curlen < a {
            curlen = a;
            continue;
        }
        curlen = a;
        while let Some((x, elem)) = cur.pop() {
            if x >= a { continue; }
            cur.push((x, elem));
            break;
        }
        let mut carry = Vec::new();
        let mut pos = a - 1;
        loop {
            if let Some((x, elem)) = cur.pop() {
                if x == pos {
                    let c = (elem + 1) / k;
                    carry.push((x, (elem + 1) % k));
                    if c == 0 { break; }
                } else {
                    // If k = 1, push 1 and it's over.
                    if k == 1 { return false; }
                    cur.push((x, elem));
                    carry.push((pos, 1));
                    break;
                }
            } else {
                if k == 1 { return false; }
                carry.push((pos, 1));
                break;
            }
            if pos >= 1 { pos -= 1; }
            else { return false; }
        }
        for t in carry.into_iter().rev() { cur.push(t); }
    }
    true
}

fn solve() {
    let out = std::io::stdout();
    let mut out = BufWriter::new(out.lock());
    macro_rules! puts {
        ($format:expr) => (write!(out,$format).unwrap());
        ($format:expr, $($args:expr),+) => (write!(out,$format,$($args),*).unwrap())
    }
    input! {
        n: usize,
        a: [i64; n],
    }
    let mut pass = n;
    let mut fail = 0;
    while pass - fail > 1 {
        let mid = (pass + fail) / 2;
        if ok(&a, mid) {
            pass = mid;
        } else {
            fail = mid;
        }
    }
    puts!("{}\n", pass);
}

fn main() {
    // In order to avoid potential stack overflow, spawn a new thread.
    let stack_size = 104_857_600; // 100 MB
    let thd = std::thread::Builder::new().stack_size(stack_size);
    thd.spawn(|| solve()).unwrap().join().unwrap();
}


Measured memory_bytes: 10620