#include<iostream>
#include<vector>
#include<string>
#include<algorithm>	
#include<map>
#include<set>
#include<utility>
#include<cmath>
#include<cstring>
#include<queue>
#include<stack>
#include<cstdio>
#include<sstream>
#include<iomanip>
#include<assert.h>
#define loop(i,a,b) for(int i=a;i<b;i++) 
#define rep(i,a) loop(i,0,a)
#define pb push_back
#define mt make_tuple
#define all(in) in.begin(),in.end()
#define shosu(x) fixed<<setprecision(x)
using namespace std;
//kaewasuretyuui
typedef long long ll;
typedef pair<int,int> pii;
typedef vector<ll> vi;
typedef vector<vi> vvi;
typedef vector<pii> vp;
typedef vector<vp> vvp;
typedef vector<string> vs;
typedef vector<double> vd;
typedef vector<vd> vvd;
typedef pair<int,pii> pip;
typedef vector<pip>vip;
const double PI=acos(-1);
const double EPS=1e-7;
const int inf=1e8;
const ll INF=1e16;
int dx[4]={0,1,0,-1};
int dy[4]={1,0,-1,0};
int main(){
	ll n;
	cin>>n;
	vi in(n);
	rep(i,n)cin>>in[i];
	if(n==1){
		cout<<in[0]<<endl;
		return 0;
	}
	vector<vvi>dp(2,vvi(n,vi(n,INF)));
	dp[0][1][n-1]=in[0];
	dp[1][0][n-2]=in[n-1];
	queue<tuple<ll,ll,ll,ll> >que;
	que.push(mt(0,1,n-1,in[0]));
	que.push(mt(1,0,n-2,in[n-1]));
	ll out=INF;
	while(!que.empty()){
		ll a,b,c,d;
		tie(a,b,c,d)=que.front();
		que.pop();
		if(dp[a][b][c]<d)continue;
//		cout<<a<<" "<<b<<" "<<c<<" "<<d<<endl;
		if(b==c){
			out=min(out,max(in[b],d+1));
			continue;
		}
		ll t=0;
		if(!a){
			t=max(in[b],d+1);
			if(dp[a][b+1][c]>t){
				dp[a][b+1][c]=t;
				que.push(mt(a,b+1,c,t));
			}
			t=max(in[c],d+c-b+1);
			if(dp[1][b][c-1]>t){
				dp[1][b][c-1]=t;
				que.push(mt(1,b,c-1,t));
			}
		}else{
			t=max(in[b],d+c-b+1);
			if(dp[0][b+1][c]>t){
				dp[0][b+1][c]=t;
				que.push(mt(0,b+1,c,t));
			}
			t=max(in[c],d+1);
			if(dp[a][b][c-1]>t){
				dp[a][b][c-1]=t;
				que.push(mt(a,b,c-1,t));
			}
		}
	}
	cout<<out<<endl;
}