Language: C++

using System;
using System.Collections.Generic;
using System.Linq;
using static System.Math;

namespace ConsoleApp1
{
    class Program
    {
        static void Main(string[] args)
        {
            A();
        }

        static void A()
        {
            var abc = ReadSplitInt();
            var order = abc.OrderBy(x => x).ToList();
            Println(order[0] + order[1]);
        }

        static List<long> ReadLines(long n)
        {
            var l = new List<long>();
            for (long i = 0; i < n; i++)
            {
                l.Add(ReadLong());
            }
            return l;
        }

        static IEnumerable<int> Range(int start, int stop)
        {
            if (start < 0 || stop < 0 || start > stop || (start <= 0 && stop <= 0))
                return new List<int>();

            return Enumerable.Range(start, stop - start);
        }

        static bool IsDigit(string str)
        {
            var i = 0;
            return int.TryParse(str, out i);
        }

        static int SumDigits(long num)
        {
            return num.ToString().Select(x => x.ToString()).Sum(int.Parse);
        }

        static int[] ToIntArray(string str)
        {
            return str.Select(x => x.ToString()).Select(int.Parse).ToArray();
        }

        static long Gcd(long a, long b) => b == 0 ? a : Gcd(b, a % b);
        static long Lcm(long a, long b) => a / Gcd(a, b) * b;

        static bool IsPrime(int x)
        {
            if (x <= 1 || (x != 2 && x % 2 == 0)) return false;
            if (x == 2) return true;

            for (int i = 3; i < x; i += 2)
                if (x % i == 0) return false;

            return true;
        }

        static string Read() => Console.ReadLine();
        static int ReadInt() => int.Parse(Read());
        static long ReadLong() => long.Parse(Read());

        static List<string> ReadSplit() => Read().Split().ToList();
        static List<int> ReadSplitInt() => Read().Split().Select(int.Parse).ToList();
        static List<long> ReadSplitLong() => Read().Split().Select(long.Parse).ToList();

        static void Print(object value) => Console.Write(value.ToString());
        static void Println(object value) => Console.WriteLine(value.ToString());

        static string Join<T>(IEnumerable<T> list) => string.Join("", list);
    }

    public static class MyExtensions
    {
        public static string Slice(this string str, int start = 0, int stop = 0)
        {
            if (start > str.Length || stop > str.Length || start < 0 || stop < 0)
                return "";

            if (stop == 0) stop = str.Length;
            return str.Substring(start, stop - start);
        }
    }
}

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 'static'
    4 | using static System.Math;
      |       ^~~~~~
a.cc:10:26: error: 'string' has not been declared
   10 |         static void Main(string[] args)
      |                          ^~~~~~
a.cc:10:35: error: expected ',' or '...' before 'args'
   10 |         static void Main(string[] args)
      |                                   ^~~~
a.cc:22:16: error: 'List' does not name a type
   22 |         static List<long> ReadLines(long n)
      |                ^~~~
a.cc:32:16: error: 'IEnumerable' does not name a type
   32 |         static IEnumerable<int> Range(int start, int stop)
      |                ^~~~~~~~~~~
a.cc:40:29: error: 'string' has not been declared
   40 |         static bool IsDigit(string str)
      |                             ^~~~~~
a.cc:51:19: error: expected unqualified-id before '[' token
   51 |         static int[] ToIntArray(string str)
      |                   ^
a.cc:56:44: error: invalid pure specifier (only '= 0' is allowed) before 'b'
   56 |         static long Gcd(long a, long b) => b == 0 ? a : Gcd(b, a % b);
      |                                            ^
a.cc:56:21: error: initializer specified for static member function 'static long int ConsoleApp1::Program::Gcd(long int, long int)'
   56 |         static long Gcd(long a, long b) => b == 0 ? a : Gcd(b, a % b);
      |                     ^~~
a.cc:57:44: error: invalid pure specifier (only '= 0' is allowed) before 'a'
   57 |         static long Lcm(long a, long b) => a / Gcd(a, b) * b;
      |                                            ^
a.cc:57:21: error: initializer specified for static member function 'static long int ConsoleApp1::Program::Lcm(long int, long int)'
   57 |         static long Lcm(long a, long b) => a / Gcd(a, b) * b;
      |                     ^~~
a.cc:70:16: error: 'string' does not name a type
   70 |         static string Read() => Console.ReadLine();
      |                ^~~~~~
a.cc:71:33: error: invalid pure specifier (only '= 0' is allowed) before 'int'
   71 |         static int ReadInt() => int.Parse(Read());
      |                                 ^~~
a.cc:71:20: error: initializer specified for static member function 'static int ConsoleApp1::Program::ReadInt()'
   71 |         static int ReadInt() => int.Parse(Read());
      |                    ^~~~~~~
a.cc:72:35: error: invalid pure specifier (only '= 0' is allowed) before 'long'
   72 |         static long ReadLong() => long.Parse(Read());
      |                                   ^~~~
a.cc:72:21: error: initializer specified for static member function 'static long int ConsoleApp1::Program::ReadLong()'
   72 |         static long ReadLong() => long.Parse(Read());
      |                     ^~~~~~~~
a.cc:74:16: error: 'List' does not name a type
   74 |         static List<string> ReadSplit() => Read().Split().ToList();
      |                ^~~~
a.cc:75:16: error: 'List' does not name a type
   75 |         static List<int> ReadSplitInt() => Read().Split().Select(int.Parse).ToList();
      |                ^~~~
a.cc:76:16: error: 'List' does not name a type
   76 |         static List<long> ReadSplitLong() => Read().Split().Select(long.Parse).ToList();
      |                ^~~~
a.cc:78:27: error: 'object' has not been declared
   78 |         static void Print(object value) => Console.Write(value.ToString());
      |                           ^~~~~~
a.cc:78:44: error: invalid pure specifier (only '= 0' is allowed) before 'Console'
   78 |         static void Print(object value) => Console.Write(value.ToString());
      |                                            ^~~~~~~
a.cc:78:21: error: initializer specified for static member function 'static void ConsoleApp1::Program::Print(int)'
   78 |         static void Print(object value) => Console.Write(value.ToString());
      |                     ^~~~~
a.cc:79:29: error: 'object' has not been declared
   79 |         static void Println(object value) => Console.WriteLine(value.ToString());
      |                             ^~~~~~
a.cc:79:46: error: invalid pure specifier (only '= 0' is allowed) before 'Console'
   79 |         static void Println(object value) => Console.WriteLine(value.ToString());
      |                                              ^~~~~~~
a.cc:79:21: error: initializer specified for static member function 'static void ConsoleApp1::Program::Println(int)'
   79 |         static void Println(object value) => Console.WriteLine(value.ToString());
      |                     ^~~~~~~
a.cc:81:16: error: 'string' does not name a type
   81 |         static string Join<T>(IEnumerable<T> list) => string.Join("", list);
      |                ^~~~~~
a.cc:82:6: error: expected ';' after class definition
   82 |     }
      |      ^
      |      ;
a.cc: In static member function 'static void ConsoleApp1::Program::A()':
a.cc:17:13: error: 'var' was not declared in this scope
   17 |             var abc = ReadSplitInt();
      |             ^~~
a.cc:18:17: error: expected ';' before 'order'
   18 |             var order = abc.OrderBy(x => x).ToList();
      |                 ^~~~~
a.cc:19:21: error: 'order' was not declared in this scope
   19 |             Println(order[0] + order[1]);
      |                     ^~~~~
a.cc: In static member function 'static bool ConsoleApp1::Program::IsDigit(int)':
a.cc:42:13: error: 'var' was not declared in this scope
   42 |             var i = 0;
      |             ^~~
a.cc:43:20: error: expected primary-expression before 'int'
   43 |             return int.TryParse(str, out i);
      |                    ^~~
a.cc:43:20: error: expected ';' before 'int'
a.cc:43:23: error: expected unqualified-id before '.' token
   43 |             return int.TryParse(str, out i);
      |                       ^
a.cc: In static member function 'static int ConsoleApp1::Program::SumDigits(long int)':
a.cc:48:24: error: request for member 'ToString' in 'num', which is of non-class type 'long int'
   48 |             return num.ToString().Select(x => x.ToString()).Sum(int.Parse);
      |                        ^~~~~~~~
a.cc:48:42: error: 'x' was not declared in this scope
   48 |             return num.ToString().Select(x => x.ToString()).Sum(int.Parse);
      |                                          ^
a.cc:48:45: error: expected primary-expression before '>' token
   48 |             return num.ToString().Select(x => x.ToString()).Sum(int.Parse);
      |                                             ^
a.cc:48:65: error: expected primary-expression before 'int'
   48 |             return num.ToString().Select(x => x.ToString()).Sum(int.Parse);
      |                                                                 ^~~
a.cc: At global scope:
a.cc:84:5: error: expected unqualified-id before 'public'
   84 |     public static class MyExtensions
      |     ^~~~~~