#include<iostream>
#include<cstdio>
#include<cstring>
#include <cstdlib>  
#include <cmath>   
#include<cctype>
#include<string>
#include<set>
#include <map>
#include<algorithm>
#include <functional>
#include<vector>
#include<climits>
#include<stack>
#include<queue>
#include <deque>
#include <climits>
#include <typeinfo>
#include <utility> 
#define all(x) (x).begin(),(x).end()
#define rep(i,m,n) for(int i = m;i < n;++i)
#define pb push_back
#define fore(i,a) for(auto &i:a)
#define rrep(i,m,n) for(int i = m;i >= n;--i)
#define INF INT_MAX/2
using namespace std;
using ll = long long;
using R = double;
const ll MOD = 1e9 + 7;
const ll inf = 1LL << 50;
struct edge { ll from; ll to; ll cost; };

int dp[1010][2][2];
void chmax(int &a,int b) {
	if (a == -1)a = b;
	else if (a < b) a = b;
}

int main(){
	int n;
	cin >> n;
	vector<int>v(n);
	rep(i, 0, n) {
		cin >> v[i];
	}

	memset(dp,-1,sizeof(dp));
	dp[0][0][0] = 0;
	rep(i, 0, n)rep(j,0,2)rep(k,0,2)rep(l,0,2){
		if (j == 1 && l == 1)continue;
		if (dp[i][0][k] == -1 || dp[i][j][l] == -1)continue;
		chmax(dp[i+1][1][0],dp[i][0][k]+v[i]);
		chmax(dp[i+1][0][j],dp[i][j][l]);
	}

	int ans = -1;
	rep(i, 0, 2)rep(j, 0, 2) {
		if (i == 1 && j == 1)continue;
		ans = max(ans,dp[n][i][j]);
	}
	cout << ans << endl;
	return 0;
}