/*
MindCoding 2016 Round 1
Vladimir VladimirM98 Milenkovic
*/
#include <bits/stdc++.h>
#define MID (left+right)/2
using namespace std;
const int MAX_N=100010;
const int MAX_TREE=4*MAX_N;
const int INF=2000000010;
int niz[MAX_N];
int ST[MAX_TREE];
void InitTree(int idx, int left, int right)
{
    if (left == right)
    {
        ST[idx] = niz[left];
        return;
    }
    InitTree(2*idx, left, MID);
    InitTree(2*idx+1, MID+1, right);
    ST[idx] = max(ST[2*idx], ST[2*idx+1]);
}
void Update(int idx, int x, int val, int left, int right)
{
    if (left == right)
    {
        ST[idx] = val;
        return;
    }
    if (x <= MID) Update(2*idx, x, val, left, MID);
    else Update(2*idx+1, x, val, MID+1, right);
    ST[idx] = max(ST[2*idx], ST[2*idx+1]);
}
int Query(int idx, int l, int r, int left, int right)
{
    if (l <= left && right <= r) return ST[idx];
    int ret = -INF;
    if (l <= MID) ret = max(ret, Query(2*idx, l, r, left, MID));
    if (r > MID) ret = max(ret, Query(2*idx+1, l, r, MID+1, right));
    return ret;
}
long long gcd(long long a,long long b)
{
    //printf("%lld %lld\n",a,b);
    if(b==0) return a;
    return gcd(b,a%b);
}
int pow2[20];
int main()
{
    pow2[0]=1;
    for(int i=1;i<20;i++) pow2[i]=2*pow2[i-1];
    int n;
    scanf("%d",&n);
    for(int i=1;i<=n;i++) scanf("%d",&niz[i]);
    InitTree(1,1,n);
    long long ans=-5ll*INF;
    for(int l=1;l<n;l++)
    {
        for(int logg=0;logg<19;logg++)
        {
            int cl=l+pow2[logg];
            if(cl>n) break;
            int cr=l+pow2[logg+1]-1;
            cr=min(cr,n);
            //printf("%d %d %d %d %d\n",niz[l],logg,Query(1,cl,cr,1,n));
            ans=max(ans,1ll*niz[l]-logg+Query(1,cl,cr,1,n));
        }
    }
    printf("%lld\n",ans);
    return 0;
}