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