
import java.util.Scanner;

/**
 *
 * @author cs18097
 */
public class Main {

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        // TODO code application logic here
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        int b = sc.nextInt();
        if(a > b){
            int tmp = a;
            a = b;
            b = tmp;
        }
        int c = b+a;
        if(c%2==1){
            System.out.print("IMPOSSIBLE");
        } else{
            System.out.print(c/2);
        }
    }
    
} import java.util.Scanner; Scanner java.util public class Main {

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        // TODO code application logic here
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        int b = sc.nextInt();
        if(a > b){
            int tmp = a;
            a = b;
            b = tmp;
        }
        int c = b+a;
        if(c%2==1){
            System.out.print("IMPOSSIBLE");
        } else{
            System.out.print(c/2);
        }
    }
    
} Main /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        // TODO code application logic here
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        int b = sc.nextInt();
        if(a > b){
            int tmp = a;
            a = b;
            b = tmp;
        }
        int c = b+a;
        if(c%2==1){
            System.out.print("IMPOSSIBLE");
        } else{
            System.out.print(c/2);
        }
    } main {
        // TODO code application logic here
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        int b = sc.nextInt();
        if(a > b){
            int tmp = a;
            a = b;
            b = tmp;
        }
        int c = b+a;
        if(c%2==1){
            System.out.print("IMPOSSIBLE");
        } else{
            System.out.print(c/2);
        }
    } 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){
            int tmp = a;
            a = b;
            b = tmp;
        } a > b a b {
            int tmp = a;
            a = b;
            b = tmp;
        } int tmp = a; tmp a a = b a b b = tmp b tmp int c = b+a; c b+a b a if(c%2==1){
            System.out.print("IMPOSSIBLE");
        } else{
            System.out.print(c/2);
        } c%2==1 c%2 c 2 1 {
            System.out.print("IMPOSSIBLE");
        } System.out.print("IMPOSSIBLE") System.out.print System.out System System.out "IMPOSSIBLE" {
            System.out.print(c/2);
        } System.out.print(c/2) System.out.print System.out System System.out c/2 c 2 String[] args args public class Main {

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        // TODO code application logic here
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        int b = sc.nextInt();
        if(a > b){
            int tmp = a;
            a = b;
            b = tmp;
        }
        int c = b+a;
        if(c%2==1){
            System.out.print("IMPOSSIBLE");
        } else{
            System.out.print(c/2);
        }
    }
    
} public class Main {

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        // TODO code application logic here
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        int b = sc.nextInt();
        if(a > b){
            int tmp = a;
            a = b;
            b = tmp;
        }
        int c = b+a;
        if(c%2==1){
            System.out.print("IMPOSSIBLE");
        } else{
            System.out.print(c/2);
        }
    }
    
} Main

import java.util.Scanner;

/**
 *
 * @author cs18097
 */
public class Main {

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        // TODO code application logic here
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        int b = sc.nextInt();
        if(a > b){
            int tmp = a;
            a = b;
            b = tmp;
        }
        int c = b+a;
        if(c%2==1){
            System.out.print("IMPOSSIBLE");
        } else{
            System.out.print(c/2);
        }
    }
    
}
