#include <cstdio>
#include <iostream>
#include <cstring>

using namespace std;

#define DIM 1000005

int rmq[18][DIM];
const int INF = -0x3f3f3f3f;
int N;

int main() {
    #ifndef ONLINE_JUDGE
    freopen("input.txt","r",stdin);
    #endif // ONLINE_JUDGE

    scanf("%d\n", &N);

    memset(rmq, INF, sizeof(rmq));

    for(int i = 1; i <= N; ++i) {
        scanf("%d", &rmq[0][i]);
    }

    for(int i = 1; i < 18; ++i) {
        for(int j = 1; j <= N; ++j) {
            rmq[i][j] = max(rmq[i - 1][j], rmq[i - 1][j + (1 << (i - 1))]);
        }
    }

    long long ans = INF;

    for(int i = 1; i < N; ++i) {
        for(int j = 0; j < 18; ++j) {
            ans = max(ans, (long long) rmq[0][i] + rmq[j][i + (1 << j)] - j);
        }
    }

    cout << ans << '\n';

    return 0;
}