import java.io.*; 
import java.util.*;

public class Main
{
  public static void main(String[] args) { new Main(); }
  Scanner in = new Scanner(System.in);
  PrintWriter out = new PrintWriter(System.out);
  
  int a, b;
  
  Main()
  {
    a = in.nextInt();
    b = in.nextInt();
    if ((a % 2) != (b % 2)) out.println("IMPOSSIBLE");
    else out.println((a+b)/2);
    out.close();
  }
} import java.io.*; io.* java import java.util.*; util.* java public class Main
{
  public static void main(String[] args) { new Main(); }
  Scanner in = new Scanner(System.in);
  PrintWriter out = new PrintWriter(System.out);
  
  int a, b;
  
  Main()
  {
    a = in.nextInt();
    b = in.nextInt();
    if ((a % 2) != (b % 2)) out.println("IMPOSSIBLE");
    else out.println((a+b)/2);
    out.close();
  }
} Main Scanner in = new Scanner(System.in); in new Scanner(System.in) PrintWriter out = new PrintWriter(System.out); out new PrintWriter(System.out) int a a b; b public static void main(String[] args) { new Main(); } main { new Main(); } new Main() String[] args args Main()
  {
    a = in.nextInt();
    b = in.nextInt();
    if ((a % 2) != (b % 2)) out.println("IMPOSSIBLE");
    else out.println((a+b)/2);
    out.close();
  } Main {
    a = in.nextInt();
    b = in.nextInt();
    if ((a % 2) != (b % 2)) out.println("IMPOSSIBLE");
    else out.println((a+b)/2);
    out.close();
  } a = in.nextInt() a in.nextInt() in.nextInt in b = in.nextInt() b in.nextInt() in.nextInt in if ((a % 2) != (b % 2)) out.println("IMPOSSIBLE");
    else out.println((a+b)/2); (a % 2) != (b % 2) (a % 2) a 2 (b % 2) b 2 out.println("IMPOSSIBLE") out.println out "IMPOSSIBLE" out.println((a+b)/2) out.println out (a+b)/2 (a+b) a b 2 out.close() out.close out public class Main
{
  public static void main(String[] args) { new Main(); }
  Scanner in = new Scanner(System.in);
  PrintWriter out = new PrintWriter(System.out);
  
  int a, b;
  
  Main()
  {
    a = in.nextInt();
    b = in.nextInt();
    if ((a % 2) != (b % 2)) out.println("IMPOSSIBLE");
    else out.println((a+b)/2);
    out.close();
  }
} public class Main
{
  public static void main(String[] args) { new Main(); }
  Scanner in = new Scanner(System.in);
  PrintWriter out = new PrintWriter(System.out);
  
  int a, b;
  
  Main()
  {
    a = in.nextInt();
    b = in.nextInt();
    if ((a % 2) != (b % 2)) out.println("IMPOSSIBLE");
    else out.println((a+b)/2);
    out.close();
  }
} Main
import java.io.*; 
import java.util.*;

public class Main
{
  public static void main(String[] args) { new Main(); }
  Scanner in = new Scanner(System.in);
  PrintWriter out = new PrintWriter(System.out);
  
  int a, b;
  
  Main()
  {
    a = in.nextInt();
    b = in.nextInt();
    if ((a % 2) != (b % 2)) out.println("IMPOSSIBLE");
    else out.println((a+b)/2);
    out.close();
  }
}