import java.util.Scanner;

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

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