#include <bits/stdc++.h>
using namespace std;

#define LL long long
typedef pair<int, int> pii;

template< typename T > inline void read(T &x)
{
    static char _c;
    static bool _f;

    x = 0;
    _f = 0;
    _c = getchar();

    while(_c < '0' || '9' < _c) {if(_c == '-') _f = true; _c = getchar();}
    while('0' <= _c && _c <= '9') {x = (x << 1) + (x << 3) + (_c & 15); _c = getchar();}
    if(_f) x = -x;
}

template < typename T > inline void Min(T &x, T y) {if(y < x) x = y;}
template < typename T > inline void Max(T &x, T y) {if(x < y) x = y;}


#define lowbit(x) ((x) & -(x))
#define lson l,mid,id<<1
#define rson mid+1,r,id<<1|1
#define ls id<<1
#define rs id<<1|1
#define MID(l,r) ((l)+(((r)-(l))>>1))
#define fi first
#define se second
#define mk make_pair
#define pb push_back

const int INF = 0x3f3f3f3f;
const double pi = (double) acos(-1.0);
const double eps = (double) 1e-8;
const int e5 = (int) 1e5 + 5;
const int MOD = (int) 1e9 + 7;
//const int MOD = (int) 998244353;
//int MOD = 998244353;

inline int sig(double x) {return x < -eps ? -1 : eps < x;}
LL fp(LL a, LL n, LL mod = MOD)
{
    if(n < 0) a = fp(a, mod - 2, mod), n = -n;
    LL res = 1; for(; n; n >>= 1, a = a * a % mod) if(n & 1) res = res * a % mod; return res;
}

struct Mint
{
    LL x;
    Mint() {x = 0;}
    Mint(int _x): x(_x) {}
    Mint(LL _x): x(_x) {}
    Mint operator - () const{
        return Mint(MOD - x);
    }
    Mint operator + (const Mint &rhs) const{
        LL res = x + rhs.x; if(res >= MOD) res -= MOD; return Mint(res);
    }
    Mint operator - (const Mint &rhs) const{
        LL res = x - rhs.x; if(res < 0) res += MOD; return Mint(res);
    }
    Mint operator * (const Mint &rhs) const{
        return Mint(x * rhs.x % MOD);
    }
    Mint operator / (const Mint &rhs) const{
        return Mint(x * fp(rhs.x, -1) % MOD);
    }
    Mint& operator += (const Mint &rhs) {
        x += rhs.x; if(x >= MOD) x -= MOD; return *this;
    }
    Mint& operator *= (const Mint &rhs) {
        x = (x * rhs.x) % MOD; return *this;
    }
    bool operator == (const Mint &rhs) const{
        return x == rhs.x;
    }
    bool operator != (const Mint &rhs) const{
        return x != rhs.x;
    }
    friend ostream& operator << (ostream &out, const Mint &rhs){
        return out << rhs.x;
    }
    friend istream& operator >> (istream &in, Mint &rhs){
        return in >> rhs.x;
    }
};


const int maxn = (int) 2e5 + 20;
const int maxm = (int) 1e6 + 20;

int n;
LL a[maxn];
LL dp[maxn][7][2];

void work()
{
    read(n);
    for(int i = 1; i <= n; i++) read(a[i]);

    int c = 3;

    for(int j = -3; j <= 3; j++)
        for(int k = 0; k < 2; k++)
            dp[0][j + c][k] = -1e18;
    dp[0][c][0] = 0;
    for(int i = 1; i <= n; i++)
    {
        for(int j = -3; j <= 3; j++)
            for(int k = 0; k < 2; k++)
                dp[i][j + c][k] = -1e18;

        int x = ((i - 1) / 2) - i / 2;
        for(int j = -3; j <= 3; j++)
        {
            if(j + c + x >= 0)
            {
                Max(dp[i][j + c + x][0], dp[i - 1][j + c][1]);
                Max(dp[i][j + c + x][0], dp[i - 1][j + c][0]);
            }

            if(j + c + x + 1 <= 6)
            {
                Max(dp[i][j + c + x + 1][1], dp[i - 1][j + c][0] + a[i]);
            }
        }

//        cout <<i <<" " <<x <<endl;
    }

    cout <<max(dp[n][c][0], dp[n][c][1]) <<endl;
}

int main(int argc, char **argv)
{
#ifdef yukihana0416
freopen("in.txt", "r", stdin);
//freopen("out.txt", "w", stdout);
#endif // yukihana0416
//    get_prime();
//    pre();
    int tc = 1;
//    read(tc);
    for(int ca = 1; ca <= tc; ca++)
    {
//        printf("Case #%d: ", ca);
        work();
    }
    return 0;
}


Measured memory_bytes: 27068