import java.util.Scanner;

public class Main{
        public  static void main(String[] args){
                calc();
        }
        static int calc(){
                Scanner sc = new Scanner(System.in);
                int a=sc.nextInt();
                int b=sc.nextInt();
                int c,d;
                if((a+b)%2==0){
                        c=(a+b)/2;
                        if(c>0){
                                System.out.println(c);
                        return c;
                        }else{
                                d =c*(-1);
                                System.out.println(d);
                                return d;
                        }
                }else{
                        System.out.println("IMPOSSIBLE");
                }
                return 0;
        }
} import java.util.Scanner; Scanner java.util public class Main{
        public  static void main(String[] args){
                calc();
        }
        static int calc(){
                Scanner sc = new Scanner(System.in);
                int a=sc.nextInt();
                int b=sc.nextInt();
                int c,d;
                if((a+b)%2==0){
                        c=(a+b)/2;
                        if(c>0){
                                System.out.println(c);
                        return c;
                        }else{
                                d =c*(-1);
                                System.out.println(d);
                                return d;
                        }
                }else{
                        System.out.println("IMPOSSIBLE");
                }
                return 0;
        }
} Main public  static void main(String[] args){
                calc();
        } main {
                calc();
        } calc() calc String[] args args static int calc(){
                Scanner sc = new Scanner(System.in);
                int a=sc.nextInt();
                int b=sc.nextInt();
                int c,d;
                if((a+b)%2==0){
                        c=(a+b)/2;
                        if(c>0){
                                System.out.println(c);
                        return c;
                        }else{
                                d =c*(-1);
                                System.out.println(d);
                                return d;
                        }
                }else{
                        System.out.println("IMPOSSIBLE");
                }
                return 0;
        } calc {
                Scanner sc = new Scanner(System.in);
                int a=sc.nextInt();
                int b=sc.nextInt();
                int c,d;
                if((a+b)%2==0){
                        c=(a+b)/2;
                        if(c>0){
                                System.out.println(c);
                        return c;
                        }else{
                                d =c*(-1);
                                System.out.println(d);
                                return d;
                        }
                }else{
                        System.out.println("IMPOSSIBLE");
                }
                return 0;
        } 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 int c c d; d if((a+b)%2==0){
                        c=(a+b)/2;
                        if(c>0){
                                System.out.println(c);
                        return c;
                        }else{
                                d =c*(-1);
                                System.out.println(d);
                                return d;
                        }
                }else{
                        System.out.println("IMPOSSIBLE");
                } (a+b)%2==0 (a+b)%2 (a+b) a b 2 0 {
                        c=(a+b)/2;
                        if(c>0){
                                System.out.println(c);
                        return c;
                        }else{
                                d =c*(-1);
                                System.out.println(d);
                                return d;
                        }
                } c=(a+b)/2 c (a+b)/2 (a+b) a b 2 if(c>0){
                                System.out.println(c);
                        return c;
                        }else{
                                d =c*(-1);
                                System.out.println(d);
                                return d;
                        } c>0 c 0 {
                                System.out.println(c);
                        return c;
                        } System.out.println(c) System.out.println System.out System System.out c return c; c {
                                d =c*(-1);
                                System.out.println(d);
                                return d;
                        } d =c*(-1) d c*(-1) c (-1) 1 System.out.println(d) System.out.println System.out System System.out d return d; d {
                        System.out.println("IMPOSSIBLE");
                } System.out.println("IMPOSSIBLE") System.out.println System.out System System.out "IMPOSSIBLE" return 0; 0 public class Main{
        public  static void main(String[] args){
                calc();
        }
        static int calc(){
                Scanner sc = new Scanner(System.in);
                int a=sc.nextInt();
                int b=sc.nextInt();
                int c,d;
                if((a+b)%2==0){
                        c=(a+b)/2;
                        if(c>0){
                                System.out.println(c);
                        return c;
                        }else{
                                d =c*(-1);
                                System.out.println(d);
                                return d;
                        }
                }else{
                        System.out.println("IMPOSSIBLE");
                }
                return 0;
        }
} public class Main{
        public  static void main(String[] args){
                calc();
        }
        static int calc(){
                Scanner sc = new Scanner(System.in);
                int a=sc.nextInt();
                int b=sc.nextInt();
                int c,d;
                if((a+b)%2==0){
                        c=(a+b)/2;
                        if(c>0){
                                System.out.println(c);
                        return c;
                        }else{
                                d =c*(-1);
                                System.out.println(d);
                                return d;
                        }
                }else{
                        System.out.println("IMPOSSIBLE");
                }
                return 0;
        }
} Main
import java.util.Scanner;

public class Main{
        public  static void main(String[] args){
                calc();
        }
        static int calc(){
                Scanner sc = new Scanner(System.in);
                int a=sc.nextInt();
                int b=sc.nextInt();
                int c,d;
                if((a+b)%2==0){
                        c=(a+b)/2;
                        if(c>0){
                                System.out.println(c);
                        return c;
                        }else{
                                d =c*(-1);
                                System.out.println(d);
                                return d;
                        }
                }else{
                        System.out.println("IMPOSSIBLE");
                }
                return 0;
        }
}