Language: C++

using System;
using System.Collections.Generic;
using System.Linq;
using System.Linq.Expressions;
using System.IO;
using System.Text;
using System.Diagnostics;
 
using static util;
using P = pair<int, int>;
 
using Binary = System.Func<System.Linq.Expressions.ParameterExpression,
                           System.Linq.Expressions.ParameterExpression,
                           System.Linq.Expressions.BinaryExpression>;
using Unary = System.Func<System.Linq.Expressions.ParameterExpression,
                          System.Linq.Expressions.UnaryExpression>;
 
class Program {
    static StreamWriter sw = new StreamWriter(Console.OpenStandardOutput()) { AutoFlush = false };
    static Scan sc = new Scan();
    const int M = 1000000007;
    const int M2 = 998244353;
    const long LM = 1L << 60;
    const double eps = 1e-11;
    static void Main(string[] args)
    {
        int n = sc.Int;
        var s = sc.LongArr;
        long ans = 0;
        for (int i = 1; i < n; i++)
        {
            long sum = 0;
            for (int j = 1; i * j < n; j++)
            {
                sum += s[i * j];
                sum += s[n - 1 - i * j];
                if (i * j >= n - 1 - i * j && (n - 1) % i == 0) break;
                int a = n - 1 - i * j;
                if (a > i) {
                    ans = Math.Max(ans, sum);
                }
            }
        }
        Prt(ans);
        sw.Flush();
    }
 
    static void DBG(string a) => Console.WriteLine(a);
    static void DBG<T>(IEnumerable<T> a) => DBG(string.Join(" ", a));
    static void DBG(params object[] a) => DBG(string.Join(" ", a));
    static void Prt(string a) => sw.WriteLine(a);
    static void Prt<T>(IEnumerable<T> a) => Prt(string.Join(" ", a));
    static void Prt(params object[] a) => Prt(string.Join(" ", a));
}
class pair<T, U> : IComparable<pair<T, U>> {
    public T v1;
    public U v2;
    public pair() : this(default(T), default(U)) {}
    public pair(T v1, U v2) { this.v1 = v1; this.v2 = v2; }
    public int CompareTo(pair<T, U> a) {
        int c = Comparer<T>.Default.Compare(v1, a.v1);
        return c != 0 ? c : Comparer<U>.Default.Compare(v2, a.v2);
    }
    public override string ToString() => v1 + " " + v2;
    public void Deconstruct(out T a, out U b) { a = v1; b = v2; }
    public static bool operator>(pair<T, U> a, pair<T, U> b) => a.CompareTo(b) > 0;
    public static bool operator<(pair<T, U> a, pair<T, U> b) => a.CompareTo(b) < 0;
    public static bool operator>=(pair<T, U> a, pair<T, U> b) => a.CompareTo(b) >= 0;
    public static bool operator<=(pair<T, U> a, pair<T, U> b) => a.CompareTo(b) <= 0;
}
static class util {
    public static pair<T, U> make_pair<T, U>(T v1, U v2) => new pair<T, U>(v1, v2);
    public static T sq<T>(T a) => Operator<T>.Multiply(a, a);
    public static T Max<T>(params T[] a) => a.Max();
    public static T Min<T>(params T[] a) => a.Min();
    public static bool inside(int i, int j, int h, int w) => i >= 0 && i < h && j >= 0 && j < w;
    static readonly int[] dd = { 0, 1, 0, -1 };
    static readonly string dstring = "RDLU";
    public static P[] adjacents(this P p) => adjacents(p.v1, p.v2);
    public static P[] adjacents(this P p, int h, int w) => adjacents(p.v1, p.v2, h, w);
    public static pair<P, char>[] adjacents_with_str(int i, int j)
        => Enumerable.Range(0, dd.Length).Select(k => new pair<P, char>(new P(i + dd[k], j + dd[k ^ 1]), dstring[k])).ToArray();
    public static pair<P, char>[] adjacents_with_str(int i, int j, int h, int w)
        => Enumerable.Range(0, dd.Length).Select(k => new pair<P, char>(new P(i + dd[k], j + dd[k ^ 1]), dstring[k])).Where(p => inside(p.v1.v1, p.v1.v2, h, w)).ToArray();
    public static P[] adjacents(int i, int j)
        => Enumerable.Range(0, dd.Length).Select(k => new P(i + dd[k], j + dd[k ^ 1])).ToArray();
    public static P[] adjacents(int i, int j, int h, int w)
        => Enumerable.Range(0, dd.Length).Select(k => new P(i + dd[k], j + dd[k ^ 1])).Where(p => inside(p.v1, p.v2, h, w)).ToArray();
    public static void Assert(bool cond) { if (!cond) throw new Exception(); }
    public static Dictionary<T, int> compress<T>(this IEnumerable<T> a)
        => a.Distinct().OrderBy(v => v).Select((v, i) => new { v, i }).ToDictionary(p => p.v, p => p.i);
    public static Dictionary<T, int> compress<T>(params IEnumerable<T>[] a) => compress(a.Aggregate(Enumerable.Union));
    public static void swap<T>(ref T a, ref T b) where T : struct { var t = a; a = b; b = t; }
    public static void swap<T>(this IList<T> a, int i, int j) where T : struct { var t = a[i]; a[i] = a[j]; a[j] = t; }
    public static T[] copy<T>(this IList<T> a) {
        var ret = new T[a.Count];
        for (int i = 0; i < a.Count; i++) ret[i] = a[i];
        return ret;
    }
}
static class Operator<T> {
    static readonly ParameterExpression x = Expression.Parameter(typeof(T), "x");
    static readonly ParameterExpression y = Expression.Parameter(typeof(T), "y");
    public static readonly Func<T, T, T> Add = Lambda(Expression.Add);
    public static readonly Func<T, T, T> Subtract = Lambda(Expression.Subtract);
    public static readonly Func<T, T, T> Multiply = Lambda(Expression.Multiply);
    public static readonly Func<T, T, T> Divide = Lambda(Expression.Divide);
    public static readonly Func<T, T> Plus = Lambda(Expression.UnaryPlus);
    public static readonly Func<T, T> Negate = Lambda(Expression.Negate);
    public static Func<T, T, T> Lambda(Binary op) => Expression.Lambda<Func<T, T, T>>(op(x, y), x, y).Compile();
    public static Func<T, T> Lambda(Unary op) => Expression.Lambda<Func<T, T>>(op(x), x).Compile();
}
 
class Scan {
    StreamReader sr;
    public Scan() { sr = new StreamReader(Console.OpenStandardInput()); }
    public Scan(string path) { sr = new StreamReader(path); }
    public int Int => int.Parse(Str);
    public long Long => long.Parse(Str);
    public double Double => double.Parse(Str);
    public string Str => sr.ReadLine().Trim();
    public pair<T, U> Pair<T, U>() {
        T a; U b;
        Multi(out a, out b);
        return new pair<T, U>(a, b);
    }
    public P P => Pair<int, int>();
    public int[] IntArr => StrArr.Select(int.Parse).ToArray();
    public long[] LongArr => StrArr.Select(long.Parse).ToArray();
    public double[] DoubleArr => StrArr.Select(double.Parse).ToArray();
    public string[] StrArr => Str.Split(new[]{' '}, StringSplitOptions.RemoveEmptyEntries);
    bool eq<T, U>() => typeof(T).Equals(typeof(U));
    T ct<T, U>(U a) => (T)Convert.ChangeType(a, typeof(T));
    T cv<T>(string s) => eq<T, int>()    ? ct<T, int>(int.Parse(s))
                       : eq<T, long>()   ? ct<T, long>(long.Parse(s))
                       : eq<T, double>() ? ct<T, double>(double.Parse(s))
                       : eq<T, char>()   ? ct<T, char>(s[0])
                                         : ct<T, string>(s);
    public void Multi<T>(out T a) => a = cv<T>(Str);
    public void Multi<T, U>(out T a, out U b)
    { var ar = StrArr; a = cv<T>(ar[0]); b = cv<U>(ar[1]); }
    public void Multi<T, U, V>(out T a, out U b, out V c)
    { var ar = StrArr; a = cv<T>(ar[0]); b = cv<U>(ar[1]); c = cv<V>(ar[2]); }
    public void Multi<T, U, V, W>(out T a, out U b, out V c, out W d)
    { var ar = StrArr; a = cv<T>(ar[0]); b = cv<U>(ar[1]); c = cv<V>(ar[2]); d = cv<W>(ar[3]); }
    public void Multi<T, U, V, W, X>(out T a, out U b, out V c, out W d, out X e)
    { var ar = StrArr; a = cv<T>(ar[0]); b = cv<U>(ar[1]); c = cv<V>(ar[2]); d = cv<W>(ar[3]); e = cv<X>(ar[4]); }
}

Compiler output:
a.cc:1:7: error: expected nested-name-specifier before 'System'
    1 | using System;
      |       ^~~~~~
a.cc:2:7: error: expected nested-name-specifier before 'System'
    2 | using System.Collections.Generic;
      |       ^~~~~~
a.cc:3:7: error: expected nested-name-specifier before 'System'
    3 | using System.Linq;
      |       ^~~~~~
a.cc:4:7: error: expected nested-name-specifier before 'System'
    4 | using System.Linq.Expressions;
      |       ^~~~~~
a.cc:5:7: error: expected nested-name-specifier before 'System'
    5 | using System.IO;
      |       ^~~~~~
a.cc:6:7: error: expected nested-name-specifier before 'System'
    6 | using System.Text;
      |       ^~~~~~
a.cc:7:7: error: expected nested-name-specifier before 'System'
    7 | using System.Diagnostics;
      |       ^~~~~~
a.cc:9:7: error: expected nested-name-specifier before 'static'
    9 | using static util;
      |       ^~~~~~
a.cc:10:11: error: 'pair' does not name a type
   10 | using P = pair<int, int>;
      |           ^~~~
a.cc:12:16: error: 'System' does not name a type
   12 | using Binary = System.Func<System.Linq.Expressions.ParameterExpression,
      |                ^~~~~~
a.cc:15:15: error: 'System' does not name a type
   15 | using Unary = System.Func<System.Linq.Expressions.ParameterExpression,
      |               ^~~~~~
a.cc:19:12: error: 'StreamWriter' does not name a type
   19 |     static StreamWriter sw = new StreamWriter(Console.OpenStandardOutput()) { AutoFlush = false };
      |            ^~~~~~~~~~~~
a.cc:20:12: error: 'Scan' does not name a type
   20 |     static Scan sc = new Scan();
      |            ^~~~
a.cc:25:22: error: 'string' has not been declared
   25 |     static void Main(string[] args)
      |                      ^~~~~~
a.cc:25:31: error: expected ',' or '...' before 'args'
   25 |     static void Main(string[] args)
      |                               ^~~~
a.cc:48:21: error: 'string' has not been declared
   48 |     static void DBG(string a) => Console.WriteLine(a);
      |                     ^~~~~~
a.cc:48:34: error: invalid pure specifier (only '= 0' is allowed) before 'Console'
   48 |     static void DBG(string a) => Console.WriteLine(a);
      |                                  ^~~~~~~
a.cc:48:17: error: initializer specified for static member function 'static void Program::DBG(int)'
   48 |     static void DBG(string a) => Console.WriteLine(a);
      |                 ^~~
a.cc:49:17: error: variable or field 'DBG' declared void
   49 |     static void DBG<T>(IEnumerable<T> a) => DBG(string.Join(" ", a));
      |                 ^~~
a.cc:49:17: error: expected ';' at end of member declaration
   49 |     static void DBG<T>(IEnumerable<T> a) => DBG(string.Join(" ", a));
      |                 ^~~
      |                    ;
a.cc:49:20: error: expected unqualified-id before '<' token
   49 |     static void DBG<T>(IEnumerable<T> a) => DBG(string.Join(" ", a));
      |                    ^
a.cc:50:21: error: 'params' has not been declared
   50 |     static void DBG(params object[] a) => DBG(string.Join(" ", a));
      |                     ^~~~~~
a.cc:50:37: error: expected ',' or '...' before 'a'
   50 |     static void DBG(params object[] a) => DBG(string.Join(" ", a));
      |                                     ^
a.cc:50:43: error: invalid pure specifier (only '= 0' is allowed) before 'DBG'
   50 |     static void DBG(params object[] a) => DBG(string.Join(" ", a));
      |                                           ^~~
a.cc:50:17: error: initializer specified for static member function 'static void Program::DBG(int*)'
   50 |     static void DBG(params object[] a) => DBG(string.Join(" ", a));
      |                 ^~~
a.cc:51:21: error: 'string' has not been declared
   51 |     static void Prt(string a) => sw.WriteLine(a);
      |                     ^~~~~~
a.cc:51:34: error: invalid pure specifier (only '= 0' is allowed) before 'sw'
   51 |     static void Prt(string a) => sw.WriteLine(a);
      |                                  ^~
a.cc:51:17: error: initializer specified for static member function 'static void Program::Prt(int)'
   51 |     static void Prt(string a) => sw.WriteLine(a);
      |                 ^~~
a.cc:52:17: error: variable or field 'Prt' declared void
   52 |     static void Prt<T>(IEnumerable<T> a) => Prt(string.Join(" ", a));
      |                 ^~~
a.cc:52:17: error: expected ';' at end of member declaration
   52 |     static void Prt<T>(IEnumerable<T> a) => Prt(string.Join(" ", a));
      |                 ^~~
      |                    ;
a.cc:52:20: error: expected unqualified-id before '<' token
   52 |     static void Prt<T>(IEnumerable<T> a) => Prt(string.Join(" ", a));
      |                    ^
a.cc:53:21: error: 'params' has not been declared
   53 |     static void Prt(params object[] a) => Prt(string.Join(" ", a));
      |                     ^~~~~~
a.cc:53:37: error: expected ',' or '...' before 'a'
   53 |     static void Prt(params object[] a) => Prt(string.Join(" ", a));
      |                                     ^
a.cc:53:43: error: invalid pure specifier (only '= 0' is allowed) before 'Prt'
   53 |     static void Prt(params object[] a) => Prt(string.Join(" ", a));
      |                                           ^~~
a.cc:53:17: error: initializer specified for static member function 'static void Program::Prt(int*)'
   53 |     static void Prt(params object[] a) => Prt(string.Join(" ", a));
      |                 ^~~
a.cc:54:2: error: expected ';' after class definition
   54 | }
      |  ^
      |  ;
a.cc: In static member function 'static void Program::Main(int*)':
a.cc:27:17: error: 'sc' was not declared in this scope
   27 |         int n = sc.Int;
      |                 ^~
a.cc:28:9: error: 'var' was not declared in this scope
   28 |         var s = sc.LongArr;
      |         ^~~
a.cc:35:24: error: 's' was not declared in this scope
   35 |                 sum += s[i * j];
      |                        ^
a.cc:40:27: error: 'Math' was not declared in this scope
   40 |                     ans = Math.Max(ans, sum);
      |                           ^~~~
a.cc:45:9: error: 'sw' was not declared in this scope
   45 |         sw.Flush();
      |         ^~
a.cc: At global scope:
a.cc:55:7: error: 'pair' is not a class template
   55 | class pair<T, U> : IComparable<pair<T, U>> {
      |       ^~~~
a.cc:55:12: error: 'T' was not declared in this scope
   55 | class pair<T, U> : IComparable<pair<T, U>> {
      |            ^
a.cc:55:15: error: 'U' was not declared in this scope
   55 | class pair<T, U> : IComparable<pair<T, U>> {
      |               ^
a.cc:55:31: error: expected template-name before '<' token
   55 | class pair<T, U> : IComparable<pair<T, U>> {
      |                               ^
a.cc:55:31: error: expected '{' before '<' token
a.cc:55:31: error: expected unqualified-id before '<' token
a.cc:72:11: error: expected ':' before 'static'
   72 |     public static pair<T, U> make_pair<T, U>(T v1, U v2) => new pair<T, U>(v1, v2);
      |           ^~~~~~~
      |           :
a.cc:72:19: error: 'pair' is not a template
   72 |     public static pair<T, U> make_pair<T, U>(T v1, U v2) => new pair<T, U>(v1, v2);
      |                   ^~~~
a.cc:72:24: error: 'T' was not declared in this scope
   72 |     public static pair<T, U> make_pair<T, U>(T v1, U v2) => new pair<T, U>(v1, v2);
      |                        ^
a.cc:72:27: error: 'U' was not declared in this scope
   72 |     public static pair<T, U> make_pair<T, U>(T v1, U v2) => new pair<T, U>(v1, v2);
      |                           ^
a.cc:72:30: error: expected ';' at end of member declaration
   72 |     public static pair<T, U> make_pair<T, U>(T v1, U v2) => new pair<T, U>(v1, v2);
      |                              ^~~~~~~~~
      |                                       ;
a.cc:72:39: error: expected unqualified-id before '<' token
   72 |     public static pair<T, U> make_pair<T, U>(T v1, U v2) => new pair<T, U>(v1, v2);
      |                                       ^
a.cc:73:11: error: expected ':' before 'static'
   73 |     public static T sq<T>(T a) => Operator<T>.Multiply(a, a);
      |           ^~~~~~~
      |           :
a.cc:73:19: error: 'T' does not name a type
   73 |     public static T sq<T>(T a) => Operator<T>.Multiply(a, a);
      |                   ^
a.cc:74:11: error: expected ':' before 'static'
   74 |     public static T Max<T>(params T[] a) => a.Max();
      |           ^~~~~~~
      |           :
a.cc:74:19: error: 'T' does not name a type
   74 |     public static T Max<T>(params T[] a) => a.Max();
      |                   ^
a.cc:75:11: error: expected ':' before 'static'
   75 |     public static T Min<T>(params T[] a) => a.Min();
      |           ^~~~~~~
      |           :
a.cc:75:19: error: 'T' does not name a type
   75 |     public static T Min<T>(params T[] a) => a.Min();
      |                   ^
a.cc:76:11: error: expected ':' before 'static'
   76 |     public static bool inside(int i, int j, int h, int w) => i >= 0 && i < h && j >= 0 && j < w;
      |           ^~~~~~~
      |           :
a.cc:76:62: error: invalid pure specifier (only '= 0' is allowed) before 'i'
   76 |     public static bool inside(int i, int j, int h, int w) => i >= 0 && i < h && j >= 0 && j < w;
      |                                                              ^
a.cc:76:24: error: initializer specified for static member function 'static bool util::inside(int, int, int, int)'
   76 |     public static bool inside(int i, int j, int h, int w) => i >= 0 && i < h && j >= 0 && j < w;
      |                        ^~~~~~
a.cc:77:12: error: 'readonly' does not name a type
   77 |     static readonly int[] dd = { 0, 1, 0, -1 };
      |            ^~~~~~~~
a.cc:78:12: error: 'readonly' does not name a type
   78 |     static readonly string dstring = "RDLU";
      |            ^~~~~~~~
a.cc:79:11: error: expected ':' before 'static'
   79 |     public static P[] adjacents(this P p) => adjacents(p.v1, p.v2);
      |           ^~~~~~~
      |           :
a.cc:79:19: error: 'P' does not name a type
   79 |     public static P[] adjacents(this P p) => adjacents(p.v1, p.