#include "iostream"
#include "climits"
#include "list"
#include "queue"
#include "stack"
#include "set"
#include "functional"
#include "algorithm"
#include "math.h"
#include "utility"
#include "map"

using namespace std;
const long long int MOD = 1000000007;

typedef pair<int, int> P;
int N;
long long int num[2000];
long long int dis[2000];
bool flag[2000];
long long int ans=LLONG_MAX;
long long int box;
long long int bag;
long long int current;
priority_queue<long long int>Q;

int main() {
	cin >> N;
	for (int i = 0; i < N; i++) {
		cin >> num[i];
	}
	for (int i = 0; i < N; i++) {
		for (int j = 0; j < N; j++) {
			dis[j] = LLONG_MAX;
			flag[j] = true;
		}
		dis[i] = num[i];
		Q.push(dis[i] * 10000 + i);
		while (!Q.empty()&&box<N) {
			if (flag[Q.top() % 10000]) {
				current = Q.top();
				Q.pop();
				box++;
				flag[current % 10000] = false;
				for (int j = 0; j < N; j++) {
					if (i != j) {
						if (max(dis[current % 10000] + abs(j - i), num[j]) < dis[j]) {
							dis[j] = max(dis[current % 10000] + abs(j - i), num[j]);
							Q.push(dis[j] * 10000 + j);
						}
					}
				}
			}
			else {
				Q.pop();
			}
		}
		bag = 0;
		for (int j = 0; j < N; j++) {
			bag = max(bag, dis[j]);
		}
		ans = min(ans, bag);
	}
	cout << ans << endl;
	return 0;
}