Language: C++

using System;
using System.Collections;
using System.Collections.Generic;
using System.Collections.Specialized;
using System.Linq;
using System.Numerics;


namespace AtCoderWorkspace
{
    class Program
    {
        static void Main(string[] args)
        {
            var cin = new Scanner();
            var A = cin.nextInt();
            var B = cin.nextInt();



            Console.WriteLine(new int[] { A + B, A - B, A * B }.Max());
        }
    }





    class Scanner
    {
        string[] s;
        int i;

        char[] cs = new char[] { ' ' };

        public Scanner()
        {
            s = new string[0];
            i = 0;
        }

        public string next()
        {
            if (i < s.Length) return s[i++];
            string st = Console.ReadLine();
            while (st == "") st = Console.ReadLine();
            s = st.Split(cs, StringSplitOptions.RemoveEmptyEntries);
            if (s.Length == 0) return next();
            i = 0;
            return s[i++];
        }

        public int nextInt()
        {
            return int.Parse(next());
        }
        public int[] ArrayInt(int N, int add = 0)
        {
            int[] Array = new int[N];
            for (int i = 0; i < N; i++)
            {
                Array[i] = nextInt() + add;
            }
            return Array;
        }

        public long nextLong()
        {
            return long.Parse(next());
        }

        public long[] ArrayLong(int N, long add = 0)
        {
            long[] Array = new long[N];
            for (int i = 0; i < N; i++)
            {
                Array[i] = nextLong() + add;
            }
            return Array;
        }

        public double nextDouble()
        {
            return double.Parse(next());
        }


        public double[] ArrayDouble(int N, double add = 0)
        {
            double[] Array = new double[N];
            for (int i = 0; i < N; i++)
            {
                Array[i] = nextDouble() + add;
            }
            return Array;
        }
    }
}

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;
      |       ^~~~~~
a.cc:3:7: error: expected nested-name-specifier before 'System'
    3 | using System.Collections.Generic;
      |       ^~~~~~
a.cc:4:7: error: expected nested-name-specifier before 'System'
    4 | using System.Collections.Specialized;
      |       ^~~~~~
a.cc:5:7: error: expected nested-name-specifier before 'System'
    5 | using System.Linq;
      |       ^~~~~~
a.cc:6:7: error: expected nested-name-specifier before 'System'
    6 | using System.Numerics;
      |       ^~~~~~
a.cc:13:26: error: 'string' has not been declared
   13 |         static void Main(string[] args)
      |                          ^~~~~~
a.cc:13:35: error: expected ',' or '...' before 'args'
   13 |         static void Main(string[] args)
      |                                   ^~~~
a.cc:23:6: error: expected ';' after class definition
   23 |     }
      |      ^
      |      ;
a.cc: In static member function 'static void AtCoderWorkspace::Program::Main(int*)':
a.cc:15:13: error: 'var' was not declared in this scope
   15 |             var cin = new Scanner();
      |             ^~~
a.cc:16:17: error: expected ';' before 'A'
   16 |             var A = cin.nextInt();
      |                 ^
a.cc:17:17: error: expected ';' before 'B'
   17 |             var B = cin.nextInt();
      |                 ^
a.cc:21:13: error: 'Console' was not declared in this scope
   21 |             Console.WriteLine(new int[] { A + B, A - B, A * B }.Max());
      |             ^~~~~~~
a.cc:21:43: error: 'A' was not declared in this scope
   21 |             Console.WriteLine(new int[] { A + B, A - B, A * B }.Max());
      |                                           ^
a.cc:21:47: error: 'B' was not declared in this scope
   21 |             Console.WriteLine(new int[] { A + B, A - B, A * B }.Max());
      |                                               ^
a.cc: At global scope:
a.cc:31:9: error: 'string' does not name a type
   31 |         string[] s;
      |         ^~~~~~
a.cc:34:13: error: expected unqualified-id before '[' token
   34 |         char[] cs = new char[] { ' ' };
      |             ^
a.cc:36:15: error: expected ':' before 'Scanner'
   36 |         public Scanner()
      |               ^~~~~~~~
      |               :
a.cc:42:15: error: expected ':' before 'string'
   42 |         public string next()
      |               ^~~~~~~
      |               :
a.cc:42:16: error: 'string' does not name a type
   42 |         public string next()
      |                ^~~~~~
a.cc:53:15: error: expected ':' before 'int'
   53 |         public int nextInt()
      |               ^~~~
      |               :
a.cc:57:15: error: expected ':' before 'int'
   57 |         public int[] ArrayInt(int N, int add = 0)
      |               ^~~~
      |               :
a.cc:57:19: error: expected unqualified-id before '[' token
   57 |         public int[] ArrayInt(int N, int add = 0)
      |                   ^
a.cc:67:15: error: expected ':' before 'long'
   67 |         public long nextLong()
      |               ^~~~~
      |               :
a.cc:72:15: error: expected ':' before 'long'
   72 |         public long[] ArrayLong(int N, long add = 0)
      |               ^~~~~
      |               :
a.cc:72:20: error: expected unqualified-id before '[' token
   72 |         public long[] ArrayLong(int N, long add = 0)
      |                    ^
a.cc:82:15: error: expected ':' before 'double'
   82 |         public double nextDouble()
      |               ^~~~~~~
      |               :
a.cc:88:15: error: expected ':' before 'double'
   88 |         public double[] ArrayDouble(int N, double add = 0)
      |               ^~~~~~~
      |               :
a.cc:88:22: error: expected unqualified-id before '[' token
   88 |         public double[] ArrayDouble(int N, double add = 0)
      |                      ^
a.cc:97:6: error: expected ';' after class definition
   97 |     }
      |      ^
      |      ;
a.cc: In constructor 'AtCoderWorkspace::Scanner::Scanner()':
a.cc:38:13: error: 's' was not declared in this scope
   38 |             s = new string[0];
      |             ^
a.cc:38:21: error: 'string' does not name a type
   38 |             s = new string[0];
      |                     ^~~~~~
a.cc: In member function 'int AtCoderWorkspace::Scanner::nextInt()':
a.cc:55:20: error: expected primary-expression before 'int'
   55 |             return int.Parse(next());
      |                    ^~~
a.cc:55:20: error: expected ';' before 'int'
a.cc:55:23: error: expected unqualified-id before '.' token
   55 |             return int.Parse(next());
      |                       ^
a.cc: In member function 'long int AtCoderWorkspace::Scanner::nextLong()':
a.cc:69:20: error: expected primary-expression before 'long'
   69 |             return long.Parse(next());
      |                    ^~~~
a.cc:69:20: error: expected ';' before 'long'
a.cc:69:24: error: expected unqualified-id before '.' token
   69 |             return long.Parse(next());
      |                        ^
a.cc: In member function 'double AtCoderWorkspace::Scanner::nextDouble()':
a.cc:84:20: error: expected primary-expression before 'double'
   84 |             return double.Parse(next());
      |                    ^~~~~~
a.cc:84:20: error: expected ';' before 'double'
a.cc:84:26: error: expected unqualified-id before '.' token
   84 |             return double.Parse(next());
      |                          ^