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

ll n,a[2100000],b[2100000],ans;
ll x,y;

int main(){
	freopen("transportt.in","r",stdin);
	freopen("transportt.out","w",stdout);
	scanf("%lld",&n);
	for (int i=1;i<=n;i++) scanf("%lld",&a[i]);
	for (int i=1;i<=n;i++) a[i+n]=a[i];
	for (int i=1;i<=n*2;i++){
		while(x<=y&&i-b[x]>n/2) x++;
        if(x<=y) ans=max(ans,a[i]+a[b[x]]+i-b[x]);
        while(x<=y&&a[b[y]]-b[y]<=a[i]-i) y--;
        b[++y]=i;
	}
	printf("%lld",ans);
	return 0;
}