import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.Arrays;
import java.util.NoSuchElementException;
import java.util.function.DoubleUnaryOperator;
import java.util.function.IntUnaryOperator;
import java.util.function.LongBinaryOperator;
import java.util.function.LongUnaryOperator;


public class Main {
    public static void main(String[] args) {
        StringBuilder out = new StringBuilder();
        solve(out);
        PrintWriter pw = new PrintWriter(System.out);
        pw.println(out);
        pw.flush();
        pw.close();
    }

    public static void solve(StringBuilder out) {
        int n = In.ni();
        long[] a = In.nl(n);
        long[][][] dp = new long[3][2][n];
        for (int i = 0; i < 3; i++) {
            for (int j = 0; j < 2; j++) {
                Arrays.fill(dp[i][j], -Const.LINF);
            }
        }
        dp[0][1][0] = a[0];
        dp[1][0][0] = 0;
        for (int i = 1; i < n; i++) {
            dp[0][0][i] = dp[0][1][i - 1];
            dp[0][1][i] = dp[0][0][i - 1] + a[i];
            dp[1][0][i] = Math.max(dp[0][0][i - 1], dp[1][1][i - 1]);
            dp[1][1][i] = dp[1][0][i - 1] + a[i];
            dp[2][0][i] = Math.max(dp[1][0][i - 1], dp[2][1][i - 1]);
            dp[2][1][i] = dp[2][0][i - 1] + a[i];
        }
        long zm = Longs.max(dp[0][0][n - 1], dp[0][1][n - 1]);
        long om = Longs.max(dp[1][0][n - 1], dp[1][1][n - 1]);
        long tm = Longs.max(dp[2][0][n - 1], dp[2][1][n - 1]);
        out.append(n % 2 == 0 ? Longs.max(zm, om) : Longs.max(om, tm));
    }
}


/**
 * @author https://atcoder.jp/users/suisen
 */
class In {
    public static final FastScanner fsc = new FastScanner();

    public static int ni() {
        return fsc.nextInt();
    }

    public static int ni(IntUnaryOperator f) {
        return f.applyAsInt(fsc.nextInt());
    }

    public static int[] ni(int n) {
        int[] a = new int[n];
        Arrays.setAll(a, i -> fsc.nextInt());
        return a;
    }

    public static int[] ni (int n, IntUnaryOperator f) {
        int[] a = new int[n];
        Arrays.setAll(a, i -> ni(f));
        return a;
    }

    public static int[][] ni(int n, int m) {
        int[][] a = new int[n][m];
        Arrays.setAll(a, i -> ni(m));
        return a;
    }

    public static int[][] ni(int n, int m, IntUnaryOperator f) {
        int[][] a = new int[n][m];
        Arrays.setAll(a, i -> ni(m, f));
        return a;
    }

    public static long nl() {
        return fsc.nextLong();
    }

    public static long nl(LongUnaryOperator f) {
        return f.applyAsLong(fsc.nextLong());
    }

    public static long[] nl(int n) {
        long[] a = new long[n];
        Arrays.setAll(a, i -> fsc.nextLong());
        return a;
    }

    public static long[] nl(int n, LongUnaryOperator f) {
        long[] a = new long[n];
        Arrays.setAll(a, i -> nl(f));
        return a;
    }

    public static long[][] nl(int n, int m) {
        long[][] a = new long[n][m];
        Arrays.setAll(a, i -> nl(m));
        return a;
    }

    public static long[][] nl(int n, int m, LongUnaryOperator f) {
        long[][] a = new long[n][m];
        Arrays.setAll(a, i -> nl(m, f));
        return a;
    }

    public static char[] nc() {
        return fsc.next().toCharArray();
    }

    public static char[][] nc(int n, int m) {
        char[][] c = new char[n][m];
        Arrays.setAll(c, i -> nc());
        return c;
    }

    public static double nd() {
        return fsc.nextDouble();
    }

    public static double nd(DoubleUnaryOperator f) {
        return f.applyAsDouble(fsc.nextDouble());
    }

    public static double[] nd(int n) {
        double[] a = new double[n];
        Arrays.setAll(a, i -> fsc.nextDouble());
        return a;
    }

    public static double[] nd(int n, DoubleUnaryOperator f) {
        double[] a = new double[n];
        Arrays.setAll(a, i -> nd(f));
        return a;
    }

    public static double[][] nd(int n, int m) {
        double[][] a = new double[n][m];
        Arrays.setAll(a, i -> nd(m));
        return a;
    }

    public static double[][] nd(int n, int m, DoubleUnaryOperator f) {
        double[][] a = new double[n][m];
        Arrays.setAll(a, i -> nd(m, f));
        return a;
    }

    public static String ns() {
        return fsc.next();
    }

    public static String[] ns(int n) {
        String[] s = new String[n];
        Arrays.setAll(s, i -> ns());
        return s;
    }

    public static boolean[][] grid(int h, int w, char trueCharacter) {
        boolean[][] grid = new boolean[h][w];
        for (int i = 0; i < h; i++) {
            char[] s = fsc.next().toCharArray();
            for (int j = 0; j < w; j++) {
                grid[i][j] = s[j] == trueCharacter;
            }
        }
        return grid;
    }
}


class FastScanner {
    private final InputStream in = System.in;
    private final byte[] buffer = new byte[1024];
    private int ptr = 0;
    private int buflen = 0;

    private boolean hasNextByte() {
        if (ptr < buflen) {
            return true;
        } else {
            ptr = 0;
            try {
                buflen = in.read(buffer);
            } catch (IOException e) {
                e.printStackTrace();
            }
            if (buflen <= 0) {
                return false;
            }
        }
        return true;
    }

    private int readByte() {
        if (hasNextByte()) {
            return buffer[ptr++];
        } else {
            return -1;
        }
    }

    private static boolean isPrintableChar(int c) {
        return 33 <= c && c <= 126;
    }

    public boolean hasNext() {
        while (hasNextByte() && !isPrintableChar(buffer[ptr])) {
            ptr++;
        }
        return hasNextByte();
    }

    public String next() {
        if (!hasNext()) {
            throw new NoSuchElementException();
        }
        StringBuilder sb = new StringBuilder();
        int b = readByte();
        while (isPrintableChar(b)) {
            sb.appendCodePoint(b);
            b = readByte();
        }
        return sb.toString();
    }

    public long nextLong() {
        if (!hasNext()) {
            throw new NoSuchElementException();
        }
        long n = 0;
        boolean minus = false;
        int b = readByte();
        if (b == '-') {
            minus = true;
            b = readByte();
        }
        if (b < '0' || '9' < b) {
            throw new NumberFormatException();
        }
        while (true) {
            if ('0' <= b && b <= '9') {
                n *= 10;
                n += b - '0';
            } else if (b == -1 || !isPrintableChar(b)) {
                return minus ? -n : n;
            } else {
                throw new NumberFormatException();
            }
            b = readByte();
        }
    }

    public int nextInt() {
        long nl = nextLong();
        if (nl < Integer.MIN_VALUE || nl > Integer.MAX_VALUE) {
            throw new NumberFormatException();
        }
        return (int) nl;
    }

    public double nextDouble() {
        return Double.parseDouble(next());
    }
}

/**
 * @author https://atcoder.jp/users/suisen
 */
class Const {
    public static final long MOD7 = 1_000_000_007;
    public static final long MOD9 = 1_000_000_009;
    public static final long MOD99 = 998_244_353;

    public static final long LINF = Long.MAX_VALUE >> 2;
    public static final int IINF = Integer.MAX_VALUE >> 1;
    public static final double DINF = 1e150;

    public static final double SDELTA = 1e-12;
    public static final double DELTA = 1e-9;
    public static final double LDELTA = 1e-6;

    public static final String YES = "YES";
    public static final String NO = "NO";
    public static final String Yes = "Yes";
    public static final String No = "No";
    public static final String POSSIBLE = "POSSIBLE";
    public static final String IMPOSSIBLE = "IMPOSSIBLE";
    public static final String Possible = "Possible";
    public static final String Impossible = "Impossible";

    public static final int[] dx8 = {1, 0, -1, 0, 1, -1, -1, 1};
    public static final int[] dy8 = {0, 1, 0, -1, 1, 1, -1, -1};
    public static final int[] dx = {1, 0, -1, 0};
    public static final int[] dy = {0, 1, 0, -1};

    private Const(){}
}


/**
 * @author https://atcoder.jp/users/suisen
 */
class Longs {
    private static final char[] hex = {'0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'a', 'b', 'c', 'd', 'e', 'f'};

    private Longs(){}

    public static long max(long... a) {
        long ret = Long.MIN_VALUE;
        for (long e : a) {
            ret = Math.max(ret, e);
        }
        return ret;
    }

    public static long min(long... a) {
        long ret = Long.MAX_VALUE;
        for (long e : a) {
            ret = Math.min(ret, e);
        }
        return ret;
    }

    public static long reduce(LongBinaryOperator func, long... a) {
        long ret = a[0];
        for (int i = 1; i < a.length; i++) {
            ret = func.applyAsLong(ret, a[i]);
        }
        return ret;
    }

    public static boolean isPowerOfTwo(long n) {
        return n != 0 && (-n & n) == n;
    }

    public static int ceilExponent(long n) {
        return 63 - Long.numberOfLeadingZeros(n) + (isPowerOfTwo(n) ? 0 : 1);
    }

    public static int floorExponent(long n) {
        return 63 - Long.numberOfLeadingZeros(n) + (n == 0 ? 1 : 0);
    }

    public static int ceilExponent(long n, int base) {
        if (base == 2) {
            return ceilExponent(n);
        }
        int i = 0;
        long m = 1;
        while (m < n) {
            i++;
            long r = m * base;
            if ((m | base) >> 31 != 0 && r / base != m) {
                break;
            }
            m = r;
        }
        return i;
    }

    /**
     * Caluculate the ceil of a/b. Returns the smallest integer greater than or
     * equal to a/b while 'a/b' rounds fractional parts to ZERO.
     * 
     * @param a
     * @param b
     * @return the smallest integer greater than or equal to a/b
     */
    public static long cld(long a, long b) {
        if (a > 0 && b > 0) return (a + b - 1) / b;
        else if (a < 0 && b < 0) return (a + b + 1) / b;
        else return a / b;
    }

    /**
     * Caluculate the floor of a/b. Returns the largest integer less than or equal
     * to a/b while 'a/b' rounds fractional parts to ZERO.
     * 
     * @param a
     * @param b
     * @return the largest integer less than or equal to a/b
     */
    public static long fld(long a, long b) {
        if (a <= 0 && b > 0) return (a - b + 1) / b;
        else if (a > 0 && b <= 0) return (a - b - 1) / b;
        else return a / b;
    }

    public static String join(String sep, long... a) {
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < a.length; i++) {
            sb.append(a[i]);
            if (i < a.length - 1) {
                sb.append(sep);
            }
        }
        return sb.toString();
    }

    public static String toBinaryString(long n) {
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < Long.SIZE; i++, n >>= 1) {
            sb.append(n & 1);
            if ((i & 3) == 3) {
                sb.append('_');
            }
        }
        return sb.reverse().toString();
    }

    public static String toHexString(long n) {
        StringBuilder sb = new StringBuilder();
        for (int i = 0, t = Long.SIZE >> 2; i < t; i++, n >>= 4) {
            sb.append(hex[(int) n & 0xf]);
            if ((i & 3) == 3) {
                sb.append('_');
            }
        }
        return sb.reverse().toString();
    }
}


Measured memory_bytes: 51008