
import java.util.Scanner;

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

import java.util.Scanner;

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