import java.util.Scanner;

public class Main {

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

	public static void main( String[] args ) {	
          
		Scanner input = new Scanner (System.in);
		
		
		int a = input.nextInt();
		
		int b = input.nextInt();
		
		int sum = a+b;
		
		if (sum%2==0) {
			
			System.out.println(sum/2);
		}
		else {
			System.out.println("IMPOSSIBLE");
		}
		
		input.close();
		
	} 
	
} Main public static void main( String[] args ) {	
          
		Scanner input = new Scanner (System.in);
		
		
		int a = input.nextInt();
		
		int b = input.nextInt();
		
		int sum = a+b;
		
		if (sum%2==0) {
			
			System.out.println(sum/2);
		}
		else {
			System.out.println("IMPOSSIBLE");
		}
		
		input.close();
		
	} main {	
          
		Scanner input = new Scanner (System.in);
		
		
		int a = input.nextInt();
		
		int b = input.nextInt();
		
		int sum = a+b;
		
		if (sum%2==0) {
			
			System.out.println(sum/2);
		}
		else {
			System.out.println("IMPOSSIBLE");
		}
		
		input.close();
		
	} Scanner input = new Scanner (System.in); input new Scanner (System.in) int a = input.nextInt(); a input.nextInt() input.nextInt input int b = input.nextInt(); b input.nextInt() input.nextInt input int sum = a+b; sum a+b a b if (sum%2==0) {
			
			System.out.println(sum/2);
		}
		else {
			System.out.println("IMPOSSIBLE");
		} sum%2==0 sum%2 sum 2 0 {
			
			System.out.println(sum/2);
		} System.out.println(sum/2) System.out.println System.out System System.out sum/2 sum 2 {
			System.out.println("IMPOSSIBLE");
		} System.out.println("IMPOSSIBLE") System.out.println System.out System System.out "IMPOSSIBLE" input.close() input.close input String[] args args public class Main {

	public static void main( String[] args ) {	
          
		Scanner input = new Scanner (System.in);
		
		
		int a = input.nextInt();
		
		int b = input.nextInt();
		
		int sum = a+b;
		
		if (sum%2==0) {
			
			System.out.println(sum/2);
		}
		else {
			System.out.println("IMPOSSIBLE");
		}
		
		input.close();
		
	} 
	
} public class Main {

	public static void main( String[] args ) {	
          
		Scanner input = new Scanner (System.in);
		
		
		int a = input.nextInt();
		
		int b = input.nextInt();
		
		int sum = a+b;
		
		if (sum%2==0) {
			
			System.out.println(sum/2);
		}
		else {
			System.out.println("IMPOSSIBLE");
		}
		
		input.close();
		
	} 
	
} Main
import java.util.Scanner;

public class Main {

	public static void main( String[] args ) {	
          
		Scanner input = new Scanner (System.in);
		
		
		int a = input.nextInt();
		
		int b = input.nextInt();
		
		int sum = a+b;
		
		if (sum%2==0) {
			
			System.out.println(sum/2);
		}
		else {
			System.out.println("IMPOSSIBLE");
		}
		
		input.close();
		
	} 
	
}
