#include <bits/stdc++.h>
#define FOREACH(it,c) for( __typeof((c).begin()) it=(c).begin();it!=(c).end();it++)
#define FOR(a,b,c) for(int a=(b);a<=(c);++a)
#define ROF(a,b,c) for(int a=(b);a>=(c);--a)
#define dbg(x) cout<<#x<<" = "<<(x)<<"\n";
#define pii pair<int,int>
#define pll pair< ll, ll >
#define pull pair< ull, ull >
#define mp make_pair
#define pb push_back
#define fi first
#define se second
#define ll long long
#define ull unsigned long long
#define tata NULL
using namespace std;
const int NMAX = 100004;
int a[NMAX],n, LOG[NMAX],dp[NMAX][18];
inline int Query(int X,int Y)
{
    int d = Y-X+1;
    int log = LOG[d];
    return max(dp[X][log],dp[X+d-(1<<log)][log]);
}
int main()
{
    #ifndef ONLINE_JUDGE
        freopen("data.in","r",stdin);
    #endif // ONLINE_JUDGE
    cin.sync_with_stdio(false);
    cin.tie(tata);
    cin >> n;
    FOR(i,1,n)
        cin >> a[i];
    FOR(i,1,n)
        dp[i][0]  = a[i];
    FOR(i,2,n)
        LOG[i] = LOG[i>>1]+1;

    for(int j = 1;(1<<j)<=n; ++j)
        for(int i = 1;i + (1 << j)-1<= n; ++i)
            dp[i][j] = max(dp[i][j-1],dp[i+(1<<(j-1))][j-1]);
    long long sol = -(1LL<<60);
    FOR(i,1,n-1)
        sol = max(sol,1LL*a[i]+a[i+1]);
    for(int len=1;(1<<len)<=n;++len)
    {
        int minlen = 1<<len;
        int maxlen = (1<<(len+1))-1;
        for(int i=minlen+1;i<=n;++i)
        {
            int x = Query(max(1,i-maxlen),i-minlen);
            sol = max(sol,1LL*a[i]+x-len);
        }
    }
    cout<<sol<<"\n";
    return 0;
}