#define _USE_MATH_DEFINES
#include <algorithm>
#include <cstdio>
#include <functional>
#include <iostream>
#include <cfloat>
#include <climits>
#include <cstdlib>
#include <cstring>
#include <cmath>
#include <map>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <time.h>
#include <vector>
#include <random>
#include <fstream>
using namespace std;

#define rep(i, N) for (int i = 0; i < N; i++)
#define pb push_back

typedef long long ll;
typedef unsigned long long ull;
typedef pair<int, int> i_i;
typedef pair<ll, int> ll_i;
typedef pair<int, ll> i_ll;
typedef pair<double, int> d_i;
typedef pair<ll, ll> ll_ll;
typedef pair<double, double> d_d;
struct edge { int v; ll w; };

const int MOD = 1000000007;
const int _MOD = 1000000009;
double EPS = 1e-10;
const ll INF = LLONG_MAX / 10;

int main() {
	int N; cin >> N;
	vector<int> a(N);
	rep(i, N) cin >> a[i];
	if (N == 1) {
		cout << a[0] << endl;
		return 0;
	}
	int lb = -1, ub = 1010000000;
	while (ub - lb > 1) {
		int mid = (lb + ub) / 2;
		bool ok = false;
		rep(i0, N) for (int d0 = -1; d0 <= 1; d0 += 2) {
			vector<int> b(N, INT_MIN);
			int i = i0, d = d0;
			rep(t, N * 2) {
				if (b[i] == INT_MIN) b[i] = mid - t;
				if (i + d < 0 || i + d >= N) d *= -1;
				i += d;
			}
			bool unko = false;
			rep(i, N) if (b[i] < a[i]) unko = true;
			if (!unko) ok = true;
		}
		if (ok) ub = mid;
		else lb = mid;
	}
	cout << ub << endl;
}