#include<bits/stdc++.h>
using namespace std;
long long n,x,ans,a[2000005],b[2000005],c[2000005];
long long d,q[2000005],l,r;
int main(){
//	freopen("transportt.in","r",stdin);
//	freopen("transportt.out","w",stdout);
	cin>>n;
	for(int i=1;i<=n;i++){
		cin>>a[i];
		b[i]=a[i]-i;
		c[i]=a[i]+i;
		a[i+n]=a[i];
		b[i+n]=b[i];
		c[i+n]=c[i];
	}
	for(int i=1;i<=2*n;i++){
		while(l<=r&&(i-q[l]>n/2))l++;
		if(l<=r)ans=max(ans,a[i]+a[q[l]]+i-q[l]);
		while(l<=r&&a[q[r]]-q[r]<=a[i]-i)r--;
		q[++r]=i;
	}
	cout<<ans;
	return 0;
}