Language: C++

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;
using Library;
using static Library.Input;

namespace AtCoderTemplate
{
    class ProgramA
    {
        static void Main()
        {
            int n = NextInt, k = NextInt;
            var s = Line.ToCharArray();
            s[k - 1] = s[k - 1].ToString().ToLower()[0];
            System.Console.WriteLine(string.Join("", s));
        }
    }
}

namespace Library
{
    class Input
    {
        static IEnumerator<string> enumerator = new string[] { }.AsEnumerable().GetEnumerator();

        public static string Line => Console.ReadLine();

        public static string[] StrArr => Line.Split(' ');

        public static int NextInt => int.Parse(NextWord());

        public static long NextLong => long.Parse(NextWord());

        public static List<int> IntList => StrArr.Select(int.Parse).ToList();

        public static List<long> LongList => StrArr.Select(long.Parse).ToList();

        public static IEnumerable<long[]> TakeLine(int N)
        {
            return Enumerable.Repeat(0, N).Select(_ => Console.ReadLine().Split(' ').Select(long.Parse).ToArray());
        }

        public static string NextWord()
        {
            while (!enumerator.MoveNext())
            {
                enumerator = StrArr.AsEnumerable().GetEnumerator();
            }
            return enumerator.Current;
        }
    }
}


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.Text.RegularExpressions;
      |       ^~~~~~
a.cc:5:7: error: expected nested-name-specifier before 'Library'
    5 | using Library;
      |       ^~~~~~~
a.cc:6:7: error: expected nested-name-specifier before 'static'
    6 | using static Library.Input;
      |       ^~~~~~
a.cc:19:6: error: expected ';' after class definition
   19 |     }
      |      ^
      |      ;
a.cc: In static member function 'static void AtCoderTemplate::ProgramA::Main()':
a.cc:14:21: error: 'NextInt' was not declared in this scope
   14 |             int n = NextInt, k = NextInt;
      |                     ^~~~~~~
a.cc:15:13: error: 'var' was not declared in this scope
   15 |             var s = Line.ToCharArray();
      |             ^~~
a.cc:16:13: error: 's' was not declared in this scope
   16 |             s[k - 1] = s[k - 1].ToString().ToLower()[0];
      |             ^
a.cc:16:15: error: 'k' was not declared in this scope
   16 |             s[k - 1] = s[k - 1].ToString().ToLower()[0];
      |               ^
a.cc:17:13: error: 'System' was not declared in this scope
   17 |             System.Console.WriteLine(string.Join("", s));
      |             ^~~~~~
a.cc:17:38: error: 'string' was not declared in this scope
   17 |             System.Console.WriteLine(string.Join("", s));
      |                                      ^~~~~~
a.cc: At global scope:
a.cc:26:16: error: 'IEnumerator' does not name a type
   26 |         static IEnumerator<string> enumerator = new string[] { }.AsEnumerable().GetEnumerator();
      |                ^~~~~~~~~~~
a.cc:26:65: error: expected unqualified-id before '.' token
   26 |         static IEnumerator<string> enumerator = new string[] { }.AsEnumerable().GetEnumerator();
      |                                                                 ^
a.cc:28:15: error: expected ':' before 'static'
   28 |         public static string Line => Console.ReadLine();
      |               ^~~~~~~
      |               :
a.cc:28:23: error: 'string' does not name a type
   28 |         public static string Line => Console.ReadLine();
      |                       ^~~~~~
a.cc:30:15: error: expected ':' before 'static'
   30 |         public static string[] StrArr => Line.Split(' ');
      |               ^~~~~~~
      |               :
a.cc:30:23: error: 'string' does not name a type
   30 |         public static string[] StrArr => Line.Split(' ');
      |                       ^~~~~~
a.cc:32:15: error: expected ':' before 'static'
   32 |         public static int NextInt => int.Parse(NextWord());
      |               ^~~~~~~
      |               :
a.cc:32:36: error: expected primary-expression before '>' token
   32 |         public static int NextInt => int.Parse(NextWord());
      |                                    ^
a.cc:32:38: error: expected primary-expression before 'int'
   32 |         public static int NextInt => int.Parse(NextWord());
      |                                      ^~~
a.cc:32:27: error: ISO C++ forbids in-class initialization of non-const static member 'Library::Input::NextInt'
   32 |         public static int NextInt => int.Parse(NextWord());
      |                           ^~~~~~~
a.cc:32:36: error: expected ';' at end of member declaration
   32 |         public static int NextInt => int.Parse(NextWord());
      |                                    ^
      |                                     ;
a.cc:32:41: error: expected unqualified-id before '.' token
   32 |         public static int NextInt => int.Parse(NextWord());
      |                                         ^
a.cc:34:15: error: expected ':' before 'static'
   34 |         public static long NextLong => long.Parse(NextWord());
      |               ^~~~~~~
      |               :
a.cc:34:38: error: expected primary-expression before '>' token
   34 |         public static long NextLong => long.Parse(NextWord());
      |                                      ^
a.cc:34:40: error: expected primary-expression before 'long'
   34 |         public static long NextLong => long.Parse(NextWord());
      |                                        ^~~~
a.cc:34:28: error: ISO C++ forbids in-class initialization of non-const static member 'Library::Input::NextLong'
   34 |         public static long NextLong => long.Parse(NextWord());
      |                            ^~~~~~~~
a.cc:34:38: error: expected ';' at end of member declaration
   34 |         public static long NextLong => long.Parse(NextWord());
      |                                      ^
      |                                       ;
a.cc:34:44: error: expected unqualified-id before '.' token
   34 |         public static long NextLong => long.Parse(NextWord());
      |                                            ^
a.cc:36:15: error: expected ':' before 'static'
   36 |         public static List<int> IntList => StrArr.Select(int.Parse).ToList();
      |               ^~~~~~~
      |               :
a.cc:36:23: error: 'List' does not name a type
   36 |         public static List<int> IntList => StrArr.Select(int.Parse).ToList();
      |                       ^~~~
a.cc:38:15: error: expected ':' before 'static'
   38 |         public static List<long> LongList => StrArr.Select(long.Parse).ToList();
      |               ^~~~~~~
      |               :
a.cc:38:23: error: 'List' does not name a type
   38 |         public static List<long> LongList => StrArr.Select(long.Parse).ToList();
      |                       ^~~~
a.cc:40:15: error: expected ':' before 'static'
   40 |         public static IEnumerable<long[]> TakeLine(int N)
      |               ^~~~~~~
      |               :
a.cc:40:23: error: 'IEnumerable' does not name a type
   40 |         public static IEnumerable<long[]> TakeLine(int N)
      |                       ^~~~~~~~~~~
a.cc:45:15: error: expected ':' before 'static'
   45 |         public static string NextWord()
      |               ^~~~~~~
      |               :
a.cc:45:23: error: 'string' does not name a type
   45 |         public static string NextWord()
      |                       ^~~~~~
a.cc:53:6: error: expected ';' after class definition
   53 |     }
      |      ^
      |      ;