#include <bits/stdc++.h>
#define x first
#define X first
#define y second
#define Y second
#define pb push_back
#define PB push_back

using namespace std;

typedef long long ll;
typedef pair<int,int> pii;
typedef vector<int> vi;
typedef vector<ll> vl;

const int N = 2e5 + 2;
const int INF = 0x3f3f3f3f;
const int MOD = 998244353;
const char en='\n';
const ll LLINF=1ll<<60;

int n, h[N];

bool possible(int L) {
	bool state = 0;
	vector<pii> v = {};
	
	for (int i = 0; i < n; ++i) {
		if (state) {
			// trebam
			int l = v[0].first, r = v[0].second;
			v.clear();
			if (h[i] < l) {
				state = 0;
				if (h[i] + 1 < l) v.push_back({h[i] + 1, l - 1});
				if (r < L) v.push_back({r + 1, L});
			} else {
				int d = h[i] - l + 1;
				if (l - d <= 0) return 0;
				v.push_back({l - d, l - 1});
			}
		} else {
			// dajem
			int tot = 0;
			for (pii p : v) {
				tot += p.second - p.first + 1;
			}
			if (h[i] > L + tot) {
				int d = h[i] - L - tot;
				state = 1;
				if (L - d + 1 <= 0) return 0;
				v = {{L - d + 1, L}};
			} else {
				int lo = 0, hi = L;
				
				while (lo < hi) {
					int mid = (lo + hi) >> 1;
					int cnt = mid;
					
					for (pii p : v) {
						if (p.second <= mid) cnt += p.second - p.first + 1;
						else if (p.first <= mid) cnt += mid - p.first + 1;
					}
					
					if (cnt >= h[i]) hi = mid;
					else lo = mid + 1;
				}
				
				if (lo < L) v = {{lo + 1, L}};
			}
		}
		/*
		cout << i << ": " << state << " ";
		for (pii p : v) {
			cout << "(" << p.first << " " << p.second << ") ";
		} cout << "\n"; */
	}
	
	return (!state || v.empty());
}

int main() {
	ios_base::sync_with_stdio(0);
	cin.tie(0);
	
	cin >> n;
	for (int i = 0; i < n; ++i) cin >> h[i];
	
	
	//cout << possible(7) << "\n";
	
	int lo = 0, hi = 1e9;
	
	while (lo < hi) {
		int mid = (lo + hi) >> 1;
		if (possible(mid)) hi = mid;
		else lo = mid + 1;
	}
	
	cout << lo << "\n";
	
	return 0;
}