Language: C++

using System;
using System.Linq;
using System.Collections.Generic;
using System.IO;

namespace atcorder2
{
    class Program
    {
        public static long Read()
        {
            return long.Parse(Console.ReadLine());
        }

        public static long[] Reads()
        {
            return Console.ReadLine().Split().Select(long.Parse).ToArray();
        }


        static void Main(string[] args)
        {
            var sw = new StreamWriter(Console.OpenStandardOutput()) { AutoFlush = false };
            Console.SetOut(sw);

            var x = Read();
            var list = new List<long>();
            list.Add(1);
            for (int i = 1; i<=x ; i++)
            {
                for (int j = 2; j*j <=i ; j++)
                {
                    var tmp = i;
                    while (tmp > 1 && tmp % j == 0)
                    {
                        tmp /= j;
                    }
                    if (tmp == 1)
                        list.Add(i);
                }
            }
            Console.WriteLine(list.Max());



 
           
            Console.Out.Flush();
        }
    }
}

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.Linq;
      |       ^~~~~~
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.IO;
      |       ^~~~~~
a.cc:10:15: error: expected ':' before 'static'
   10 |         public static long Read()
      |               ^~~~~~~
      |               :
a.cc:15:15: error: expected ':' before 'static'
   15 |         public static long[] Reads()
      |               ^~~~~~~
      |               :
a.cc:15:27: error: expected unqualified-id before '[' token
   15 |         public static long[] Reads()
      |                           ^
a.cc:21:26: error: 'string' has not been declared
   21 |         static void Main(string[] args)
      |                          ^~~~~~
a.cc:21:35: error: expected ',' or '...' before 'args'
   21 |         static void Main(string[] args)
      |                                   ^~~~
a.cc:50:6: error: expected ';' after class definition
   50 |     }
      |      ^
      |      ;
a.cc: In static member function 'static long int atcorder2::Program::Read()':
a.cc:12:20: error: expected primary-expression before 'long'
   12 |             return long.Parse(Console.ReadLine());
      |                    ^~~~
a.cc:12:20: error: expected ';' before 'long'
a.cc:12:24: error: expected unqualified-id before '.' token
   12 |             return long.Parse(Console.ReadLine());
      |                        ^
a.cc: In static member function 'static void atcorder2::Program::Main(int*)':
a.cc:23:13: error: 'var' was not declared in this scope
   23 |             var sw = new StreamWriter(Console.OpenStandardOutput()) { AutoFlush = false };
      |             ^~~
a.cc:24:13: error: 'Console' was not declared in this scope
   24 |             Console.SetOut(sw);
      |             ^~~~~~~
a.cc:24:28: error: 'sw' was not declared in this scope
   24 |             Console.SetOut(sw);
      |                            ^~
a.cc:26:17: error: expected ';' before 'x'
   26 |             var x = Read();
      |                 ^
a.cc:27:17: error: expected ';' before 'list'
   27 |             var list = new List<long>();
      |                 ^~~~
a.cc:28:13: error: 'list' was not declared in this scope
   28 |             list.Add(1);
      |             ^~~~
a.cc:29:32: error: 'x' was not declared in this scope
   29 |             for (int i = 1; i<=x ; i++)
      |                                ^
a.cc:33:25: error: expected ';' before 'tmp'
   33 |                     var tmp = i;
      |                         ^~~
a.cc:34:28: error: 'tmp' was not declared in this scope
   34 |                     while (tmp > 1 && tmp % j == 0)
      |                            ^~~
a.cc:38:25: error: 'tmp' was not declared in this scope
   38 |                     if (tmp == 1)
      |                         ^~~