#include <bits/stdc++.h>
using namespace std;

const int N = 4000010;

int a[N],ans,n;
deque <int> q; 

int main(){
	freopen("transportt.in","r",stdin);
	freopen("transportt.out","w",stdout);
	scanf("%d",&n);
	for(int i=1;i<=n;i++){
		scanf("%d",&a[i]);
		a[i+n]=a[i];
	}
	for(int i=1;i<=2*n;i++){
		while(q.size()&&i-q.front()>n/2){
			q.pop_front();
		}
		if(q.size()){
			ans=max(ans,a[i]+a[q.front()]+i-q.front());
		}
		while(q.size()&&a[q.back()]-q.back()<=a[i]-i){
			q.pop_back();
		}
		q.push_back(i);
	}
	printf("%d",ans);
	return 0;
}