#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const int N=2000010;
ll n,a[N],ans;
deque<ll>q;
int main () {
	freopen("transportt.in","r",stdin);
	freopen("transportt.out","w",stdout);
	ios::sync_with_stdio(0);
	cin.tie(0),cout.tie(0);
	cin >> n;
	for (int i=1;i<=n;i++) {
		cin >> a[i];
		a[i+n]=a[i];
	}
    for (int i=1;i<=2*n;i++) {
    	if (!q.empty()&&i-q.front()>n/2) q.pop_front();
		if (!q.empty()) ans=max(ans,a[i]+i+a[q.front()]-q.front());
		while (!q.empty()&&a[q.back()]-q.back()<=a[i]-i) q.pop_back();
		q.push_back(i);
	}
	cout << ans <<endl;
	return 0;
}