#include <cstdio>
#include <cstring>
#include <string>
#include <iostream>
#include <cmath>
#include <bitset>
#include <vector>
#include <map>
#include <set>
#include <queue>
#include <deque>
#include <algorithm>
#include <unordered_map>
#include <unordered_set>
using namespace std;
typedef long long int ll;
typedef pair<int, int> P;
const int INF=1e9+10000;
int n;
int a[2000];
int b[2000];
int memo[2000][2000];
int solve(int i, int j){
	if(i==j) return 0;
	if(memo[i][j]) return memo[i][j];
	int ans=INF;
	if(i<j){
		int s1=solve(j, i+1);
		if(s1>=0){
			if(s1+j-i<=b[i]) ans=min(ans, s1+j-i);
		}
		int s2=solve(i+1, j);
		if(s2>=0){
			if(s2+1<=b[i]) ans=min(ans, s2+1);
		}
	}else{
		int s1=solve(j, i-1);
		if(s1>=0){
			if(s1+i-j<=b[i]) ans=min(ans, s1+i-j);
		}
		int s2=solve(i-1, j);
		if(s2>=0){
			if(s2+1<=b[i]) ans=min(ans, s2+1);
		}
	}
	if(ans==INF) return memo[i][j]=-1;
	else return memo[i][j]=ans;
}
int main()
{
	cin>>n;
	int mx=0;
	for(int i=0; i<n; i++){
		cin>>a[i];
		mx=max(mx, a[i]);
	}
	int t1=mx, t2=mx+n;
	while(t1!=t2){
		int t0=(t1+t2)/2;
		for(int i=0; i<n; i++){
			b[i]=t0-a[i];
		}
		for(int i=0; i<n; i++) for(int j=0; j<n; j++) memo[i][j]=0;
		if(solve(0, n-1)==-1 && solve(n-1, 0)==-1) t1=t0+1;
		else t2=t0;
	}
	cout<<t1<<endl;	
	return 0;
}