import java.util.*;
import java.lang.*;
import java.io.*;
 
public class Main
{
	public static void main (String[] args) throws java.lang.Exception
	{
	
		Scanner blank = new Scanner(System.in);
		int a = blank.nextInt();
		int b = blank.nextInt();
		if((a + b) % 2 == 0){
		    System.out.println((a + b) / 2);
		}
		else{
		    System.out.println("IMPOSSIBLE");
		}
	}
} import java.util.*; util.* java import java.lang.*; lang.* java import java.io.*; io.* java public class Main
{
	public static void main (String[] args) throws java.lang.Exception
	{
	
		Scanner blank = new Scanner(System.in);
		int a = blank.nextInt();
		int b = blank.nextInt();
		if((a + b) % 2 == 0){
		    System.out.println((a + b) / 2);
		}
		else{
		    System.out.println("IMPOSSIBLE");
		}
	}
} Main public static void main (String[] args) throws java.lang.Exception
	{
	
		Scanner blank = new Scanner(System.in);
		int a = blank.nextInt();
		int b = blank.nextInt();
		if((a + b) % 2 == 0){
		    System.out.println((a + b) / 2);
		}
		else{
		    System.out.println("IMPOSSIBLE");
		}
	} main {
	
		Scanner blank = new Scanner(System.in);
		int a = blank.nextInt();
		int b = blank.nextInt();
		if((a + b) % 2 == 0){
		    System.out.println((a + b) / 2);
		}
		else{
		    System.out.println("IMPOSSIBLE");
		}
	} Scanner blank = new Scanner(System.in); blank new Scanner(System.in) int a = blank.nextInt(); a blank.nextInt() blank.nextInt blank int b = blank.nextInt(); b blank.nextInt() blank.nextInt blank if((a + b) % 2 == 0){
		    System.out.println((a + b) / 2);
		}
		else{
		    System.out.println("IMPOSSIBLE");
		} (a + b) % 2 == 0 (a + b) % 2 (a + b) a b 2 0 {
		    System.out.println((a + b) / 2);
		} System.out.println((a + b) / 2) System.out.println System.out System System.out (a + b) / 2 (a + b) a b 2 {
		    System.out.println("IMPOSSIBLE");
		} System.out.println("IMPOSSIBLE") System.out.println System.out System System.out "IMPOSSIBLE" String[] args args public class Main
{
	public static void main (String[] args) throws java.lang.Exception
	{
	
		Scanner blank = new Scanner(System.in);
		int a = blank.nextInt();
		int b = blank.nextInt();
		if((a + b) % 2 == 0){
		    System.out.println((a + b) / 2);
		}
		else{
		    System.out.println("IMPOSSIBLE");
		}
	}
} public class Main
{
	public static void main (String[] args) throws java.lang.Exception
	{
	
		Scanner blank = new Scanner(System.in);
		int a = blank.nextInt();
		int b = blank.nextInt();
		if((a + b) % 2 == 0){
		    System.out.println((a + b) / 2);
		}
		else{
		    System.out.println("IMPOSSIBLE");
		}
	}
} Main
import java.util.*;
import java.lang.*;
import java.io.*;
 
public class Main
{
	public static void main (String[] args) throws java.lang.Exception
	{
	
		Scanner blank = new Scanner(System.in);
		int a = blank.nextInt();
		int b = blank.nextInt();
		if((a + b) % 2 == 0){
		    System.out.println((a + b) / 2);
		}
		else{
		    System.out.println("IMPOSSIBLE");
		}
	}
}