Language: C++

using System;
using System.Collections.Generic;
using static System.Math;
using System.Linq;
using System.Text;
 
class DandA {
    static void Main(string[] args) {
        int[] ab = ReadLineIntArray();
        int min = Math.Min(ab[0], ab[1]);
        int max = Math.Max(ab[0], ab[1]);
        if ((max - min) % 2 == 0) {
            Console.WriteLine(max - (max - min));
        } else {
            Console.WriteLine("IMPOSSIBLE");
        }
    }
 
    static string alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZABCDEFGHIJKLMNOPQRSTUVWXYZ";
    static string[] salphabet = {"a","b","c","d","e","f","g","h","i","j"};
    static string[] strAry = new string[] {"L", "R", "U", "D"};
    static int[] intArray = new int[10];
    static List<int> intList = new List<int>();
    static Dictionary<int, long> myTable = new Dictionary<int, long>();
    static HashSet<int>[] rtnArrAb = null;
    static HashSet<int>[] rtnArrCd = null;
    static long[,] Combi = new long[2001,2001];
    static long div = 1000000007;
    static int[] atest = new int[] {1, 1, 1, 2, 1, 2, 1, 5, 2, 2, 1, 5, 1, 2, 1, 14, 1, 5, 1, 5, 2, 2, 1, 15, 2, 2, 5, 4, 1, 4, 1, 51};
  
    static private string ReadLineStr() {return Console.ReadLine();}
    static private int ReadLineInt() {return int.Parse(Console.ReadLine());}
    static private long ReadLineLong() {return long.Parse(Console.ReadLine());}
    static private double ReadLineDouble() {return double.Parse(Console.ReadLine());}
    static private decimal ReadLineDecimal() {return decimal.Parse(Console.ReadLine());}
    static private string[] ReadLineStrArray() {return Console.ReadLine().Trim().Split();}
    static private int[] ReadLineIntArray() {return Console.ReadLine().Trim().Split().Select(a => int.Parse(a)).ToArray();}
    static private long[] ReadLineLongArray() {return Console.ReadLine().Trim().Split().Select(a => long.Parse(a)).ToArray();}
    static private double[] ReadLineDoubleArray() {return Console.ReadLine().Trim().Split().Select(a => double.Parse(a)).ToArray();}
    static private decimal[] ReadLineDecimalArray() {return Console.ReadLine().Trim().Split().Select(a => decimal.Parse(a)).ToArray();}
    static private List<int> ReadLineIntList() {return Console.ReadLine().Trim().Split().Select(a => int.Parse(a)).ToList();}
    static private List<long> ReadLineLongList() {return Console.ReadLine().Trim().Split().Select(a => long.Parse(a)).ToList();}
    static private string[][] ReadMLineStrArray(int arg) { return new int[arg].Select(_ => Console.ReadLine().Split()).ToArray();}
    static private int[][] ReadMLineIntArray(int arg) { return new int[arg].Select(_ => Console.ReadLine().Split().Select(int.Parse).ToArray()).ToArray();}
 
    static private bool IsPrime(int num) {
        if (num < 2) return false;
        if (num == 2) return true;
        else if (num % 2 == 0) return false;
        double sqrtNum = Math.Sqrt(num);
        for (int i = 3; i <= sqrtNum; i += 2) {
            if (num % i == 0) return false;
        }
        return true;
    }
  
    static private string IntToNString(int x, int n) {
        var nstring = "";
        var numbers = new string[] { "0", "1", "2", "3", "4", "5", "6", "7", "8", "9", "a", "b", "c", "d", "e", "f" };
        int r = 1;
        int q = x;
        while (q > 0) {
            r = q % n;
            nstring += numbers[r];
            q = q / n;
        }
        return string.Join("",nstring.Reverse());
    }
    static private long Gcd(long m,long n){
        long temp;
        while(m % n != 0) {
            temp = n;
            n = m % n;
            m = temp;
        }
        return n;
    }
 
    static private long Combination(long n,long r){
        if(n < r) {return 0;}
        if(n == r || r == 0) {return 1;}
        else if(Combi[n,r] == 0) {
            Combi[n,r] = (Combination(n-1,r-1)+Combination(n-1,r)) % div;
        }
        return Combi[n,r];
    }
}

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 'static'
    3 | using static System.Math;
      |       ^~~~~~
a.cc:4:7: error: expected nested-name-specifier before 'System'
    4 | using System.Linq;
      |       ^~~~~~
a.cc:5:7: error: expected nested-name-specifier before 'System'
    5 | using System.Text;
      |       ^~~~~~
a.cc:8:22: error: 'string' has not been declared
    8 |     static void Main(string[] args) {
      |                      ^~~~~~
a.cc:8:31: error: expected ',' or '...' before 'args'
    8 |     static void Main(string[] args) {
      |                               ^~~~
a.cc:19:12: error: 'string' does not name a type
   19 |     static string alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZABCDEFGHIJKLMNOPQRSTUVWXYZ";
      |            ^~~~~~
a.cc:20:12: error: 'string' does not name a type
   20 |     static string[] salphabet = {"a","b","c","d","e","f","g","h","i","j"};
      |            ^~~~~~
a.cc:21:12: error: 'string' does not name a type
   21 |     static string[] strAry = new string[] {"L", "R", "U", "D"};
      |            ^~~~~~
a.cc:22:15: error: expected unqualified-id before '[' token
   22 |     static int[] intArray = new int[10];
      |               ^
a.cc:23:12: error: 'List' does not name a type
   23 |     static List<int> intList = new List<int>();
      |            ^~~~
a.cc:24:12: error: 'Dictionary' does not name a type
   24 |     static Dictionary<int, long> myTable = new Dictionary<int, long>();
      |            ^~~~~~~~~~
a.cc:25:12: error: 'HashSet' does not name a type
   25 |     static HashSet<int>[] rtnArrAb = null;
      |            ^~~~~~~
a.cc:26:12: error: 'HashSet' does not name a type
   26 |     static HashSet<int>[] rtnArrCd = null;
      |            ^~~~~~~
a.cc:27:16: error: expected unqualified-id before '[' token
   27 |     static long[,] Combi = new long[2001,2001];
      |                ^
a.cc:28:17: error: ISO C++ forbids in-class initialization of non-const static member 'DandA::div'
   28 |     static long div = 1000000007;
      |                 ^~~
a.cc:29:15: error: expected unqualified-id before '[' token
   29 |     static int[] atest = new int[] {1, 1, 1, 2, 1, 2, 1, 5, 2, 2, 1, 5, 1, 2, 1, 14, 1, 5, 1, 5, 2, 2, 1, 15, 2, 2, 5, 4, 1, 4, 1, 51};
      |               ^
a.cc:31:12: error: expected unqualified-id before 'private'
   31 |     static private string ReadLineStr() {return Console.ReadLine();}
      |            ^~~~~~~
a.cc:32:12: error: expected unqualified-id before 'private'
   32 |     static private int ReadLineInt() {return int.Parse(Console.ReadLine());}
      |            ^~~~~~~
a.cc:33:12: error: expected unqualified-id before 'private'
   33 |     static private long ReadLineLong() {return long.Parse(Console.ReadLine());}
      |            ^~~~~~~
a.cc:34:12: error: expected unqualified-id before 'private'
   34 |     static private double ReadLineDouble() {return double.Parse(Console.ReadLine());}
      |            ^~~~~~~
a.cc:35:12: error: expected unqualified-id before 'private'
   35 |     static private decimal ReadLineDecimal() {return decimal.Parse(Console.ReadLine());}
      |            ^~~~~~~
a.cc:36:12: error: expected unqualified-id before 'private'
   36 |     static private string[] ReadLineStrArray() {return Console.ReadLine().Trim().Split();}
      |            ^~~~~~~
a.cc:37:12: error: expected unqualified-id before 'private'
   37 |     static private int[] ReadLineIntArray() {return Console.ReadLine().Trim().Split().Select(a => int.Parse(a)).ToArray();}
      |            ^~~~~~~
a.cc:38:12: error: expected unqualified-id before 'private'
   38 |     static private long[] ReadLineLongArray() {return Console.ReadLine().Trim().Split().Select(a => long.Parse(a)).ToArray();}
      |            ^~~~~~~
a.cc:39:12: error: expected unqualified-id before 'private'
   39 |     static private double[] ReadLineDoubleArray() {return Console.ReadLine().Trim().Split().Select(a => double.Parse(a)).ToArray();}
      |            ^~~~~~~
a.cc:40:12: error: expected unqualified-id before 'private'
   40 |     static private decimal[] ReadLineDecimalArray() {return Console.ReadLine().Trim().Split().Select(a => decimal.Parse(a)).ToArray();}
      |            ^~~~~~~
a.cc:41:12: error: expected unqualified-id before 'private'
   41 |     static private List<int> ReadLineIntList() {return Console.ReadLine().Trim().Split().Select(a => int.Parse(a)).ToList();}
      |            ^~~~~~~
a.cc:42:12: error: expected unqualified-id before 'private'
   42 |     static private List<long> ReadLineLongList() {return Console.ReadLine().Trim().Split().Select(a => long.Parse(a)).ToList();}
      |            ^~~~~~~
a.cc:43:12: error: expected unqualified-id before 'private'
   43 |     static private string[][] ReadMLineStrArray(int arg) { return new int[arg].Select(_ => Console.ReadLine().Split()).ToArray();}
      |            ^~~~~~~
a.cc:44:12: error: expected unqualified-id before 'private'
   44 |     static private int[][] ReadMLineIntArray(int arg) { return new int[arg].Select(_ => Console.ReadLine().Split().Select(int.Parse).ToArray()).ToArray();}
      |            ^~~~~~~
a.cc:46:12: error: expected unqualified-id before 'private'
   46 |     static private bool IsPrime(int num) {
      |            ^~~~~~~
a.cc:57:12: error: expected unqualified-id before 'private'
   57 |     static private string IntToNString(int x, int n) {
      |            ^~~~~~~
a.cc:69:12: error: expected unqualified-id before 'private'
   69 |     static private long Gcd(long m,long n){
      |            ^~~~~~~
a.cc:79:12: error: expected unqualified-id before 'private'
   79 |     static private long Combination(long n,long r){
      |            ^~~~~~~
a.cc:87:2: error: expected ';' after class definition
   87 | }
      |  ^
      |  ;
a.cc: In static member function 'static void DandA::Main(int*)':
a.cc:9:12: error: structured binding declaration cannot have type 'int'
    9 |         int[] ab = ReadLineIntArray();
      |            ^~
a.cc:9:12: note: type must be cv-qualified 'auto' or reference to cv-qualified 'auto'
a.cc:9:12: error: empty structured binding declaration
a.cc:9:15: error: expected initializer before 'ab'
    9 |         int[] ab = ReadLineIntArray();
      |               ^~
a.cc:10:19: error: 'Math' was not declared in this scope
   10 |         int min = Math.Min(ab[0], ab[1]);
      |                   ^~~~
a.cc:10:28: error: 'ab' was not declared in this scope
   10 |         int min = Math.Min(ab[0], ab[1]);
      |                            ^~
a.cc:13:13: error: 'Console' was not declared in this scope
   13 |             Console.WriteLine(max - (max - min));
      |             ^~~~~~~
a.cc:15:13: error: 'Console' was not declared in this scope
   15 |             Console.WriteLine("IMPOSSIBLE");
      |             ^~~~~~~