#include <iostream>
#include <cstdio>
using namespace std;
const int N=1e6+10;
int q[N],l,r,n,a[N],ans;
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);
	for(int i=1;i<n;i++)a[n+i]=a[i];
	l=1,r=0;
	for(int i=1;i<2*n;i++){
		while(l<=r&&i-q[l]>n-i+q[l])l++;
		if(l<=r)ans=max(ans,a[i]+i-q[l]+a[q[l]]);
		while(l<=r&&-q[r]+a[q[r]]<=-i+a[i])r--;
		q[++r]=i;
	}
	printf("%d\n",ans);
	return 0;
}