#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef vector<int> vint;
typedef pair<int,int> pint;
typedef vector<pint> vpint;
#define rep(i,n) for(int i=0;i<(n);i++)
#define reps(i,f,n) for(int i=(f);i<(n);i++)
#define each(it,v) for(__typeof((v).begin()) it=(v).begin();it!=(v).end();it++)
#define all(v) (v).begin(),(v).end()
#define pb push_back
#define mp make_pair
#define fi first
#define se second
#define chmax(a, b) a = (((a)<(b)) ? (b) : (a))
#define chmin(a, b) a = (((a)>(b)) ? (b) : (a))
const int MOD = 1e9 + 7;
const int INF = 1e9;
const ll INFF = 1e18;

//dp[isright][l][r] := 閉区間[l,r]が未収穫で、現在のセル(isrigt=0のときl-1, r+1)にいるための最短時刻
ll dp[2][2010][2010];
int n;
ll a[2010];
int main(void){
	cin >> n; rep(i, n) cin >> a[i];
	if(n == 1){
		printf("%lld\n", a[0]); return 0;
	}
	rep(i, 2)rep(j, 2010)rep(k, 2010) dp[i][j][k] = INF;
	dp[0][1][n - 1] = a[0];
	dp[1][0][n - 2] = a[n - 1];
	rep(i, 2){
		for(int len = n; len >= 0; --len)for(int j = 0; j + len < n; ++j){
			int k = j + len;
			if(j == k) continue;
			if(dp[i][j][k] == INF || j >= k) continue;
			ll time = dp[i][j][k];
			if(i == 0){//左端(j - 1)にいるとき
				//j-1->j
				if(time + 1 >= a[j]) chmin(dp[0][j + 1][k], time + 1);
				else chmin(dp[0][j + 1][k], a[j]);
				//j-1->k
				if(time + k - j + 1 >= a[k]) chmin(dp[1][j][k - 1], time + k - j + 1);
				else chmin(dp[1][j][k - 1], a[k]);
			}else{//右端(k + 1)にいるとき
				//k+1->k
				if(time + 1 >= a[k]) chmin(dp[1][j][k - 1], time + 1);
				else chmin(dp[1][j][k - 1], a[k]);
				//k+1->j
				if(time + k - j + 1 >= a[j]) chmin(dp[0][j + 1][k], time + k - j + 1);
				else chmin(dp[0][j + 1][k], a[j]);
			}
		}
	}
	ll ans = INF;
	rep(i, 2)rep(j, n){
		// printf("dp[%d][%d][%d] = %d\n", i, j, j, dp[i][j][j]);
		if(dp[i][j][j] + 1 >= a[j]) chmin(ans, dp[i][j][j] + 1);
		else chmin(ans, a[j]);
	}
	printf("%lld\n", ans);
	return 0;
}