#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];
bool fflag[2001];
bool gflag[2000];
long long int ff;
long long int ans=LLONG_MAX;
long long int box;
long long int bag;
long long int current;
priority_queue<long long int, vector<long long int>, greater<long long int> >Q;
long long int node;

int main() {
	cin >> N;
	for (int i = 0; i < N; i++) {
		cin >> num[i];
		gflag[i] = true;
	}
	for (int i = 0; i < N; i++) {
		if (num[i] + N < ans/*&&gflag[i]*/) {
			for (int j = 0; j < N; j++) {
				dis[j] = LLONG_MAX;
				flag[j] = true;
				fflag[j] = true;
			}
			fflag[2000] = true;
			dis[i] = num[i];
			Q.push((dis[i] * 10000 + i) * 10000 + 2000);
			box = 0;
			while (!Q.empty() && box < N) {
				if (flag[(Q.top() % 100000000) / 10000] && fflag[(Q.top() % 10000)]) {
					current = Q.top();
					node = (current % 100000000) / 10000;
					dis[node] = current / 100000000;
					if (dis[node] == num[node]) {
						gflag[node] = false;
					}
					Q.pop();
					box++;
					flag[node] = false;
					fflag[current % 10000] = false;
					for (int j = 0; j < N; j++) {
						if (flag[j]) {
							if (max(dis[node] + abs(j - node), num[j]) < dis[j]) {
								Q.push(max(dis[node] + abs(j - node), num[j]) * 100000000 + j * 10000 + node);
							}
						}
					}
				}
				else {
					Q.pop();
				}
			}
			while (!Q.empty()) {
				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;
}