import java.util.*;

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

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