// RandomUsername (Nikola Jovanovic)
// MindCoding 2016
// Round 1
// D

#include <bits/stdc++.h>
#define DBG false
#define debug(x) if(DBG) printf("(ln %d) %s = %d\n", __LINE__, #x, x);
#define lld long long
#define ff(i,a,b) for(lld i=a; i<=b; i++)
#define fb(i,a,b) for(int i=a; i>=b; i--)
#define par pair<int, int>
#define fi first
#define se second
#define mid (l+r)/2
#define INF 1000000000
#define MAXN 100005
#define MAXLOGN 20

using namespace std;

lld n;
lld a[MAXN];

lld st[MAXN][MAXLOGN];
lld pow2[MAXN];

void init()
{
    pow2[1] = 0;
    for(lld i = 2; i < MAXN; i++)
    {
        pow2[i] = pow2[i/2] + 1;
    }

    for(lld i = 1; i <= n; i++)
        st[i][0] = a[i];

    for(lld deg = 1; (1 << deg) <= n; deg++)
    {
        for(lld i = 1; i <= n; i++)
        {
            st[i][deg] = st[i][deg-1];
            lld leap = (1 << (deg-1));
            if(i + leap <= n)
                st[i][deg] = max(st[i][deg-1], st[i + leap][deg-1]);
        }
    }
}

lld get(lld a, lld b)
{
    lld bestPow = pow2[b - a + 1];
    return max( st[a][bestPow], st[b + 1 - (1 << bestPow)][bestPow] );
}

int main()
{
    scanf("%lld", &n);
    ff(i, 1, n)
        scanf("%lld", &a[i]);
    init();
    lld ret = -1;
    bool dn = false;
    ff(L, 1, n)
    {
        lld off = 1;
        ff(lg, 0, 17)
        {
            if(L+off > n) break;
            lld val = get(L+off, min(n, L+off*2-1) );
            off *= 2;

            lld curr = a[L] + val - lg;
            if(!dn)
            {
                ret = curr;
                dn = true;
            }
            else
            {
                ret = max(ret, curr);
            }
        }
    }
    printf("%lld\n", ret);
    return 0;
}