/* Aa^~ kokoro ga pyonpyon suru n jaa^~
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*/
#include<bits/stdc++.h>
using namespace std;
#ifndef ONLINE_JUDGE
#define dbg(x...) do { cout << "\033[32;1m " << #x << " -> "; err(x); } while (0)
void err() { cout << "\033[39;0m" << endl; }
template<template<typename...> class T, typename t, typename... A>
void err(T<t> a, A... x) { for (auto v: a) cout << v << ' '; err(x...); }
template<typename T, typename... A>
void err(T a, A... x) { cout << a << ' '; err(x...); }
#else
#define dbg(...)
#endif
typedef long long ll;
typedef pair<int,int> pi;
typedef vector<int> vi;
template<class T> using vc=vector<T>;
template<class T> using vvc=vc<vc<T>>;
template<class T> void mkuni(vector<T>&v)
{
    sort(v.begin(),v.end());
    v.erase(unique(v.begin(),v.end()),v.end());
}
ll rand_int(ll l, ll r) //[l, r]
{
    static mt19937_64 gen(chrono::steady_clock::now().time_since_epoch().count());
    return uniform_int_distribution<ll>(l, r)(gen);
}
template<class T>
void print(T x,int suc=1)
{
    cout<<x;
    if(suc==1) cout<<'\n';
    else cout<<' ';
}
template<class T>
void print(const vector<T>&v,int suc=1)
{
    for(int i=0;i<v.size();i++)
    print(v[i],i==(int)(v.size())-1?suc:2);
}
const int maxn=2e5+7;
ll dp[maxn][5];
int main()
{
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    memset(dp,-0x3f,sizeof(dp));
    ll inf=dp[0][0];
    int n;
    cin>>n;
    vi a(n+1);
    for(int i=1;i<=n;i++) cin>>a[i];
    const int var=2;
    dp[0][var]=dp[1][var]=0;
    dp[1][var+1]=a[1];
    for(int i=2;i<=n;i++)
    {
        for(int j=0;j<5;j++)
        {
            int cc=(i-2)/2+j-var;
            cc=cc-i/2;
            int pcc=(i-1)/2+j-var;
            pcc=pcc-i/2;
            if(pcc+var>=0&&pcc+var<5&&dp[i-1][j]!=inf)
                dp[i][pcc+var]=max(dp[i][pcc+var],dp[i-1][j]);
            cc++;
            if(cc+var>=0&&cc+var<5&&dp[i-2][j]!=inf)
                dp[i][cc+var]=max(dp[i][cc+var],dp[i-2][j]+a[i]);
            //dbg(i,j,(i/2)+j-var,dp[i][j]);
        }
    }
    print(dp[n][var]);
}

Measured memory_bytes: 11892