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

int main() {
	freopen("transportt.in","r",stdin);
		freopen("transportt.out","w",stdout);
    ios::sync_with_stdio(false);
    cin.tie(0);
    int N;
    cin >> N;
    vector<long long> A(N);
    for (int i = 0; i < N; i++) {
        cin >> A[i];
    }

    if (N == 1) {
        cout << 0 << endl;
        return 0;
    }

    int d = N / 2;
    int len = 2 * N;
    vector<long long> B(len);
    for (int i = 0; i < len; i++) {
        B[i] = A[i % N];
    }

    vector<long long> C(len);
    for (int i = 0; i < len; i++) {
        C[i] = B[i] - i;
    }

    deque<int> q;
    long long ans = 0;

    for (int j = 0; j < len; j++) {
        while (!q.empty() && q.front() < j - d) {
            q.pop_front();
        }

        if (!q.empty()) {
            long long cur = C[q.front()] + B[j] + j;
            if (cur > ans) {
                ans = cur;
            }
        }

        while (!q.empty() && C[q.back()] <= C[j]) {
            q.pop_back();
        }
        q.push_back(j);
    }

    cout << ans << endl;
    return 0;
}