#include <iostream>

using namespace std;

const int NMAX = 100000 + 1;
const int LOG_MAX = 18;
const int INF = 0x3fffffff;

int n;
int rmq[LOG_MAX][NMAX];

void citeste() {
    cin >> n;
    for (int i = 1; i <= n; i++)
        cin >> rmq[0][i];
}

void init() {
    for (int i = 1, p = 1; i < LOG_MAX; i++, p *= 2)
        for (int j = 1; j <= n; j++) {
            rmq[i][j] = max(rmq[i - 1][j], rmq[i - 1][j + p]);
        }
}

void rezolva() {
    long long sol = (-1) * INF;

    for (int i = 1; i < n; i++)
        for (int lg = 0, p = 1; lg < LOG_MAX; lg++, p *= 2) {
            sol = max(sol, 1LL * rmq[0][i] + rmq[lg][i + p] - lg);
        }

    cout << sol << '\n';
}

int main() {
    citeste();
    init();
    rezolva();
}