#include<cstdio>
#include<cstdlib>
#include<queue>
#define N 1000005
using namespace std;
int n,a[N],b[N],mid,ans;
deque<int>q;
int max(int x,int y){return x>y?x:y;}
void solve(int l,int r)
{
	if(l==r)return;
	int mid=l+r>>1;
	solve(l,mid);solve(mid+1,r);
	int sum=0;
	for(int i=l;i<=mid;i++)sum=max(sum,a[i]+mid-i);
	for(int i=mid+1;i<=r;i++)
		ans=max(ans,a[i]+sum+i-mid);
}
int read()
{
	int sum=0;bool f=0;char c=getchar();
	for(;c<48||c>57;c=getchar())if(c==45)f=1;
	for(;c>=48&&c<=57;c=getchar())sum=sum*10+(c&15);
	return f?-sum:sum; 
}
int main(){
	freopen("transportt.in","r",stdin);freopen("transportt.out","w",stdout);
	n=read();for(int i=1;i<=n;i++)a[i]=read();
	mid=1+n>>1;
	solve(1,mid);solve(mid+1,n);
	for(int i=1;i<=mid;i++)b[i]=a[i]+n+i;
	for(int i=mid+1;i<=n;i++)
	{
		while(q.size()&&b[q.back()]<=b[i-mid])q.pop_back();
		q.push_back(i-mid);
		ans=max(ans,a[i]+b[q.front()]-i);
	}
	for(int i=1;i<=mid;i++)b[i]=a[i]+mid-i;
	int sum=0;
	for(int i=n;i>mid;i--)
	{
		sum=max(sum,b[mid+i-n]);
		ans=max(ans,a[i]+sum+i-mid);
	}
	printf("%d",ans);
	return 0;
}