#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 int INF = INT_MAX / 2;

int main() {
	int N; cin >> N;
	vector<int> a(N);
	rep(i, N) cin >> a[i];
	vector<vector<int> > dpl(N + 1, vector<int>(N + 1, INF)), dpr = dpl;
	dpl[0][N] = dpr[0][N] = -1;
	for (int d = N; d >= 1; d--)
		for (int l = 0; l + d <= N; l++) {
			int r = l + d;
			dpl[l + 1][r] = min(dpl[l + 1][r], max(a[l], dpl[l][r] + 1));
			dpr[l][r - 1] = min(dpr[l][r - 1], max(a[r - 1], dpr[l][r] + 1));
			dpr[l][r - 1] = min(dpr[l][r - 1], max(a[r - 1], dpl[l][r] + r - l));
			dpl[l + 1][r] = min(dpl[l + 1][r], max(a[l], dpr[l][r] + r - l));
		}
	int ans = INF;
	rep(i, N) ans = min(ans, min(dpl[i][i], dpr[i][i]));
	cout << ans << endl;
}