
import java.util.*;

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

import java.util.*;

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