import java.util.*;

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

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