Language: C

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

namespace AtCoder
{
    public class Program
    {
        public static void Main(string[] args)
        {
            new Program().Solve(new ConsoleInput(Console.In, ' '));
        }

        public void Solve(ConsoleInput cin)
        {
            var S = cin.Read;
            var ans = 0;
            for(int i=0;i<S.Length/2; ++i){
                if(S[i]!=S[S.Length-1-i])ans++;
            }
            WriteLine(ans);
        }
    }

    public class ConsoleInput
    {
        private readonly System.IO.TextReader _stream;
        private char _separator = ' ';
        private Queue<string> inputStream;
        public ConsoleInput(System.IO.TextReader stream, char separator = ' ')
        {
            this._separator = separator;
            this._stream = stream;
            inputStream = new Queue<string>();
        }
        public string Read
        {
            get
            {
                if (inputStream.Count != 0) return inputStream.Dequeue();
                string[] tmp = _stream.ReadLine().Split(_separator);
                for (int i = 0; i < tmp.Length; ++i)
                    inputStream.Enqueue(tmp[i]);
                return inputStream.Dequeue();
            }
        }
        public string ReadLine { get { return _stream.ReadLine(); }}
        public int ReadInt { get { return int.Parse(Read); }}
        public long ReadLong { get { return long.Parse(Read); }}
        public double ReadDouble { get { return double.Parse(Read); }}
        public string[] ReadStrArray(long N) { var ret = new string[N]; for (long i = 0; i < N; ++i) ret[i] = Read; return ret;}
        public int[] ReadIntArray(long N) { var ret = new int[N]; for (long i = 0; i < N; ++i) ret[i] = ReadInt; return ret;}
        public long[] ReadLongArray(long N) { var ret = new long[N]; for (long i = 0; i < N; ++i) ret[i] = ReadLong; return ret;}
    }
}


Compiler output:
main.c:1:1: error: unknown type name 'using'
    1 | using System;
      | ^~~~~
main.c:2:1: error: unknown type name 'using'
    2 | using System.Text;
      | ^~~~~
main.c:2:13: error: expected '=', ',', ';', 'asm' or '__attribute__' before '.' token
    2 | using System.Text;
      |             ^
main.c:3:1: error: unknown type name 'using'
    3 | using System.Linq;
      | ^~~~~
main.c:3:13: error: expected '=', ',', ';', 'asm' or '__attribute__' before '.' token
    3 | using System.Linq;
      |             ^
main.c:4:1: error: unknown type name 'using'
    4 | using System.Collections;
      | ^~~~~
main.c:4:13: error: expected '=', ',', ';', 'asm' or '__attribute__' before '.' token
    4 | using System.Collections;
      |             ^
main.c:5:1: error: unknown type name 'using'
    5 | using System.Collections.Generic;
      | ^~~~~
main.c:5:13: error: expected '=', ',', ';', 'asm' or '__attribute__' before '.' token
    5 | using System.Collections.Generic;
      |             ^
main.c:6:6: error: expected ';' before 'static'
    6 | using static System.Console;
      |      ^~~~~~~
      |      ;
main.c:6:20: error: expected '=', ',', ';', 'asm' or '__attribute__' before '.' token
    6 | using static System.Console;
      |                    ^
main.c:7:6: error: expected ';' before 'static'
    7 | using static System.Math;
      |      ^~~~~~~
      |      ;
main.c:7:20: error: expected '=', ',', ';', 'asm' or '__attribute__' before '.' token
    7 | using static System.Math;
      |                    ^
main.c:9:1: error: unknown type name 'namespace'
    9 | namespace AtCoder
      | ^~~~~~~~~
main.c:10:1: error: expected '=', ',', ';', 'asm' or '__attribute__' before '{' token
   10 | {
      | ^