Language: C

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using static System.Math;
using System.Text;
using System.Threading;
using System.Globalization;
using System.Runtime.CompilerServices;
using Library;

namespace Program
{
    public static class ABC162F
    {
        static public int numberOfRandomCases = 0;
        static public void MakeTestCase(List<string> _input, List<string> _output, ref Func<string[], bool> _outputChecker)
        {
        }
        static public void Solve()
        {
            var N = NN;
            var AList = NNList(N);
            var dp = new long[N + 1, 2, 3];
            for (var i = 0; i <= N; i++)
            {
                for (var j = 0; j < 2; j++)
                {
                    for (var k = 0; k < 3; k++)
                    {
                        dp[i, j, k] = long.MinValue / 4;
                    }
                }
            }
            dp[0, 0, 0] = 0;
            for (var i = 0; i < N; i++)
            {
                for (var k = 0; k < 3; k++)
                {
                    dp[i + 1, 1, k] = dp[i, 0, k] + AList[i];
                    if (k < 2) dp[i + 1, 0, k + 1] = dp[i, 0, k];
                    dp[i + 1, 0, k] = Max(dp[i + 1, 0, k], dp[i, 1, k]);
                }
            }
            var ret = long.MinValue;
            if (N % 2 == 1)
            {
                ret = Max(ret, dp[N - 2, 1, 0]);
                ret = Max(ret, dp[N - 1, 1, 1]);
                ret = Max(ret, dp[N - 0, 1, 2]);
            }
            else
            {
                ret = Max(ret, dp[N - 1, 1, 0]);
                ret = Max(ret, dp[N - 0, 1, 1]);
            }
            Console.WriteLine(ret);
        }
        class Printer : StreamWriter
        {
            public override IFormatProvider FormatProvider { get { return CultureInfo.InvariantCulture; } }
            public Printer(Stream stream) : base(stream, new UTF8Encoding(false, true)) { base.AutoFlush = false; }
            public Printer(Stream stream, Encoding encoding) : base(stream, encoding) { base.AutoFlush = false; }
        }
        static LIB_FastIO fastio = new LIB_FastIODebug();
        static public void Main(string[] args) { if (args.Length == 0) { fastio = new LIB_FastIO(); Console.SetOut(new Printer(Console.OpenStandardOutput())); } var t = new Thread(Solve, 134217728); t.Start(); t.Join(); Console.Out.Flush(); }
        static long NN => fastio.Long();
        static double ND => fastio.Double();
        static string NS => fastio.Scan();
        static long[] NNList(long N) => Repeat(0, N).Select(_ => NN).ToArray();
        static double[] NDList(long N) => Repeat(0, N).Select(_ => ND).ToArray();
        static string[] NSList(long N) => Repeat(0, N).Select(_ => NS).ToArray();
        static long Count<T>(this IEnumerable<T> x, Func<T, bool> pred) => Enumerable.Count(x, pred);
        static IEnumerable<T> Repeat<T>(T v, long n) => Enumerable.Repeat<T>(v, (int)n);
        static IEnumerable<int> Range(long s, long c) => Enumerable.Range((int)s, (int)c);
        static IOrderedEnumerable<T> OrderByRand<T>(this IEnumerable<T> x) => Enumerable.OrderBy(x, _ => xorshift);
        static IOrderedEnumerable<T> OrderBy<T>(this IEnumerable<T> x) => Enumerable.OrderBy(x.OrderByRand(), e => e);
        static IOrderedEnumerable<T1> OrderBy<T1, T2>(this IEnumerable<T1> x, Func<T1, T2> selector) => Enumerable.OrderBy(x.OrderByRand(), selector);
        static IOrderedEnumerable<T> OrderByDescending<T>(this IEnumerable<T> x) => Enumerable.OrderByDescending(x.OrderByRand(), e => e);
        static IOrderedEnumerable<T1> OrderByDescending<T1, T2>(this IEnumerable<T1> x, Func<T1, T2> selector) => Enumerable.OrderByDescending(x.OrderByRand(), selector);
        static IOrderedEnumerable<string> OrderBy(this IEnumerable<string> x) => x.OrderByRand().OrderBy(e => e, StringComparer.OrdinalIgnoreCase);
        static IOrderedEnumerable<T> OrderBy<T>(this IEnumerable<T> x, Func<T, string> selector) => x.OrderByRand().OrderBy(selector, StringComparer.OrdinalIgnoreCase);
        static IOrderedEnumerable<string> OrderByDescending(this IEnumerable<string> x) => x.OrderByRand().OrderByDescending(e => e, StringComparer.OrdinalIgnoreCase);
        static IOrderedEnumerable<T> OrderByDescending<T>(this IEnumerable<T> x, Func<T, string> selector) => x.OrderByRand().OrderByDescending(selector, StringComparer.OrdinalIgnoreCase);
        static uint xorshift { get { _xsi.MoveNext(); return _xsi.Current; } }
        static IEnumerator<uint> _xsi = _xsc();
        static IEnumerator<uint> _xsc() { uint x = 123456789, y = 362436069, z = 521288629, w = (uint)(DateTime.Now.Ticks & 0xffffffff); while (true) { var t = x ^ (x << 11); x = y; y = z; z = w; w = (w ^ (w >> 19)) ^ (t ^ (t >> 8)); yield return w; } }
    }
}namespace Library {
    class LIB_FastIO
    {
        public LIB_FastIO() { str = Console.OpenStandardInput(); }
        readonly Stream str;
        readonly byte[] buf = new byte[1024];
        int len, ptr;
        public bool isEof = false;
        public bool IsEndOfStream { get { return isEof; } }
        byte read()
        {
            if (isEof) throw new EndOfStreamException();
            if (ptr >= len)
            {
                ptr = 0;
                if ((len = str.Read(buf, 0, 1024)) <= 0)
                {
                    isEof = true;
                    return 0;
                }
            }
            return buf[ptr++];
        }
        char Char()
        {
            byte b = 0;
            do b = read();
            while (b < 33 || 126 < b);
            return (char)b;
        }
        virtual public string Scan()
        {
            var sb = new StringBuilder();
            for (var b = Char(); b >= 33 && b <= 126; b = (char)read())
                sb.Append(b);
            return sb.ToString();
        }
        virtual public long Long()
        {
            long ret = 0; byte b = 0; var ng = false;
            do b = read();
            while (b != '-' && (b < '0' || '9' < b));
            if (b == '-') { ng = true; b = read(); }
            for (; true; b = read())
            {
                if (b < '0' || '9' < b)
                    return ng ? -ret : ret;
                else ret = ret * 10 + b - '0';
            }
        }
        virtual public double Double() { return double.Parse(Scan(), CultureInfo.InvariantCulture); }
    }
    class LIB_FastIODebug : LIB_FastIO
    {
        Queue<string> param = new Queue<string>();
        string NextString() { if (param.Count == 0) foreach (var item in Console.ReadLine().Split(' ')) param.Enqueue(item); return param.Dequeue(); }
        public LIB_FastIODebug() { }
        public override string Scan() => NextString();
        public override long Long() => long.Parse(NextString());
        public override double Double() => double.Parse(NextString());
    }
}


Compiler output:
main.c:1:1: error: unknown type name 'using'
    1 | using System;
      | ^~~~~
main.c:2:1: error: unknown type name 'using'
    2 | using System.Collections.Generic;
      | ^~~~~
main.c:2:13: error: expected '=', ',', ';', 'asm' or '__attribute__' before '.' token
    2 | using System.Collections.Generic;
      |             ^
main.c:3:1: error: unknown type name 'using'
    3 | using System.IO;
      | ^~~~~
main.c:3:13: error: expected '=', ',', ';', 'asm' or '__attribute__' before '.' token
    3 | using System.IO;
      |             ^
main.c:4:1: error: unknown type name 'using'
    4 | using System.Linq;
      | ^~~~~
main.c:4:13: error: expected '=', ',', ';', 'asm' or '__attribute__' before '.' token
    4 | using System.Linq;
      |             ^
main.c:5:6: error: expected ';' before 'static'
    5 | using static System.Math;
      |      ^~~~~~~
      |      ;
main.c:5:20: error: expected '=', ',', ';', 'asm' or '__attribute__' before '.' token
    5 | using static System.Math;
      |                    ^
main.c:6:1: error: unknown type name 'using'
    6 | using System.Text;
      | ^~~~~
main.c:6:13: error: expected '=', ',', ';', 'asm' or '__attribute__' before '.' token
    6 | using System.Text;
      |             ^
main.c:7:1: error: unknown type name 'using'
    7 | using System.Threading;
      | ^~~~~
main.c:7:13: error: expected '=', ',', ';', 'asm' or '__attribute__' before '.' token
    7 | using System.Threading;
      |             ^
main.c:8:1: error: unknown type name 'using'
    8 | using System.Globalization;
      | ^~~~~
main.c:8:13: error: expected '=', ',', ';', 'asm' or '__attribute__' before '.' token
    8 | using System.Globalization;
      |             ^
main.c:9:1: error: unknown type name 'using'
    9 | using System.Runtime.CompilerServices;
      | ^~~~~
main.c:9:13: error: expected '=', ',', ';', 'asm' or '__attribute__' before '.' token
    9 | using System.Runtime.CompilerServices;
      |             ^
main.c:10:1: error: unknown type name 'using'
   10 | using Library;
      | ^~~~~
main.c:12:1: error: unknown type name 'namespace'
   12 | namespace Program
      | ^~~~~~~~~
main.c:13:1: error: expected '=', ',', ';', 'asm' or '__attribute__' before '{' token
   13 | {
      | ^
main.c:89:2: error: unknown type name 'namespace'
   89 | }namespace Library {
      |  ^~~~~~~~~
main.c:89:20: error: expected '=', ',', ';', 'asm' or '__attribute__' before '{' token
   89 | }namespace Library {
      |                    ^