Language: C++

using System;
using System.Collections.Generic;


namespace ConsoleApp_CS
{
    class Program
    {
        static void Main(string[] args)
        {
            var a = CIn.ReadInt();
            var b = CIn.ReadInt();
            a = a + b;
            if (a >= 10)
            {
                Console.WriteLine("error");
            }
            else
            {
                Console.WriteLine(a);
            }
        }
    }

    class CIn
    {
        private static readonly Queue<string> fieldBuffer = new Queue<string>();

        public static string Read()
        {
            if (fieldBuffer.Count == 0)
            {
                foreach (var field in Console.ReadLine().Split())
                {
                    fieldBuffer.Enqueue(field);
                }
            }
            return fieldBuffer.Dequeue();
        }

        public static int ReadInt() => int.Parse(Read());
        public static long ReadLong() => long.Parse(Read());
        public static double ReadDouble() => double.Parse(Read());

        public static string[] ReadStringArray(long n)
        {
            var array = new string[n];
            for (var i = 0; i < n; i++)
            {
                array[i] = Read();
            }
            return array;
        }

        public static int[] ReadIntArray(long n)
        {
            var array = new int[n];
            for (var i = 0; i < n; i++)
            {
                array[i] = ReadInt();
            }
            return array;
        }

        public static long[] ReadLongArray(long n)
        {
            var array = new long[n];
            for (var i = 0; i < n; i++)
            {
                array[i] = ReadLong();
            }
            return array;
        }

        public static double[] ReadDoubleArray(long n)
        {
            var array = new double[n];
            for (var i = 0; i < n; i++)
            {
                array[i] = ReadDouble();
            }
            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.Generic;
      |       ^~~~~~
a.cc:9:26: error: 'string' has not been declared
    9 |         static void Main(string[] args)
      |                          ^~~~~~
a.cc:9:35: error: expected ',' or '...' before 'args'
    9 |         static void Main(string[] args)
      |                                   ^~~~
a.cc:23:6: error: expected ';' after class definition
   23 |     }
      |      ^
      |      ;
a.cc: In static member function 'static void ConsoleApp_CS::Program::Main(int*)':
a.cc:11:13: error: 'var' was not declared in this scope
   11 |             var a = CIn.ReadInt();
      |             ^~~
a.cc:12:17: error: expected ';' before 'b'
   12 |             var b = CIn.ReadInt();
      |                 ^
a.cc:13:13: error: 'a' was not declared in this scope
   13 |             a = a + b;
      |             ^
a.cc:13:21: error: 'b' was not declared in this scope
   13 |             a = a + b;
      |                     ^
a.cc:16:17: error: 'Console' was not declared in this scope
   16 |                 Console.WriteLine("error");
      |                 ^~~~~~~
a.cc:20:17: error: 'Console' was not declared in this scope
   20 |                 Console.WriteLine(a);
      |                 ^~~~~~~
a.cc: At global scope:
a.cc:27:16: error: expected ':' before 'static'
   27 |         private static readonly Queue<string> fieldBuffer = new Queue<string>();
      |                ^~~~~~~
      |                :
a.cc:27:24: error: 'readonly' does not name a type
   27 |         private static readonly Queue<string> fieldBuffer = new Queue<string>();
      |                        ^~~~~~~~
a.cc:29:15: error: expected ':' before 'static'
   29 |         public static string Read()
      |               ^~~~~~~
      |               :
a.cc:29:23: error: 'string' does not name a type
   29 |         public static string Read()
      |                       ^~~~~~
a.cc:41:15: error: expected ':' before 'static'
   41 |         public static int ReadInt() => int.Parse(Read());
      |               ^~~~~~~
      |               :
a.cc:41:40: error: invalid pure specifier (only '= 0' is allowed) before 'int'
   41 |         public static int ReadInt() => int.Parse(Read());
      |                                        ^~~
a.cc:41:27: error: initializer specified for static member function 'static int ConsoleApp_CS::CIn::ReadInt()'
   41 |         public static int ReadInt() => int.Parse(Read());
      |                           ^~~~~~~
a.cc:42:15: error: expected ':' before 'static'
   42 |         public static long ReadLong() => long.Parse(Read());
      |               ^~~~~~~
      |               :
a.cc:42:42: error: invalid pure specifier (only '= 0' is allowed) before 'long'
   42 |         public static long ReadLong() => long.Parse(Read());
      |                                          ^~~~
a.cc:42:28: error: initializer specified for static member function 'static long int ConsoleApp_CS::CIn::ReadLong()'
   42 |         public static long ReadLong() => long.Parse(Read());
      |                            ^~~~~~~~
a.cc:43:15: error: expected ':' before 'static'
   43 |         public static double ReadDouble() => double.Parse(Read());
      |               ^~~~~~~
      |               :
a.cc:43:46: error: invalid pure specifier (only '= 0' is allowed) before 'double'
   43 |         public static double ReadDouble() => double.Parse(Read());
      |                                              ^~~~~~
a.cc:43:30: error: initializer specified for static member function 'static double ConsoleApp_CS::CIn::ReadDouble()'
   43 |         public static double ReadDouble() => double.Parse(Read());
      |                              ^~~~~~~~~~
a.cc:45:15: error: expected ':' before 'static'
   45 |         public static string[] ReadStringArray(long n)
      |               ^~~~~~~
      |               :
a.cc:45:23: error: 'string' does not name a type
   45 |         public static string[] ReadStringArray(long n)
      |                       ^~~~~~
a.cc:55:15: error: expected ':' before 'static'
   55 |         public static int[] ReadIntArray(long n)
      |               ^~~~~~~
      |               :
a.cc:55:26: error: expected unqualified-id before '[' token
   55 |         public static int[] ReadIntArray(long n)
      |                          ^
a.cc:65:15: error: expected ':' before 'static'
   65 |         public static long[] ReadLongArray(long n)
      |               ^~~~~~~
      |               :
a.cc:65:27: error: expected unqualified-id before '[' token
   65 |         public static long[] ReadLongArray(long n)
      |                           ^
a.cc:75:15: error: expected ':' before 'static'
   75 |         public static double[] ReadDoubleArray(long n)
      |               ^~~~~~~
      |               :
a.cc:75:29: error: expected unqualified-id before '[' token
   75 |         public static double[] ReadDoubleArray(long n)
      |                             ^
a.cc:84:6: error: expected ';' after class definition
   84 |     }
      |      ^
      |      ;