import java.util.Scanner;
public class Main {
	public static void main(String[] args) {


        // Scannerクラスのインスタンスの生成
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        int b = sc.nextInt();
        int k = (a + b) / 2;

        if ((a - k) == -(b - k) || -(a - k) == (b - k)) {
        	System.out.println(k);

        } else {
        	System.out.println("IMPOSSIBLE");
        }



	}



} import java.util.Scanner; Scanner java.util public class Main {
	public static void main(String[] args) {


        // Scannerクラスのインスタンスの生成
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        int b = sc.nextInt();
        int k = (a + b) / 2;

        if ((a - k) == -(b - k) || -(a - k) == (b - k)) {
        	System.out.println(k);

        } else {
        	System.out.println("IMPOSSIBLE");
        }



	}



} Main public static void main(String[] args) {


        // Scannerクラスのインスタンスの生成
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        int b = sc.nextInt();
        int k = (a + b) / 2;

        if ((a - k) == -(b - k) || -(a - k) == (b - k)) {
        	System.out.println(k);

        } else {
        	System.out.println("IMPOSSIBLE");
        }



	} main {


        // Scannerクラスのインスタンスの生成
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        int b = sc.nextInt();
        int k = (a + b) / 2;

        if ((a - k) == -(b - k) || -(a - k) == (b - k)) {
        	System.out.println(k);

        } else {
        	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 int k = (a + b) / 2; k (a + b) / 2 (a + b) a b 2 if ((a - k) == -(b - k) || -(a - k) == (b - k)) {
        	System.out.println(k);

        } else {
        	System.out.println("IMPOSSIBLE");
        } (a - k) == -(b - k) || -(a - k) == (b - k) (a - k) == -(b - k) (a - k) a k -(b - k) (b - k) b k -(a - k) == (b - k) -(a - k) (a - k) a k (b - k) b k {
        	System.out.println(k);

        } System.out.println(k) System.out.println System.out System System.out k {
        	System.out.println("IMPOSSIBLE");
        } 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) {


        // Scannerクラスのインスタンスの生成
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        int b = sc.nextInt();
        int k = (a + b) / 2;

        if ((a - k) == -(b - k) || -(a - k) == (b - k)) {
        	System.out.println(k);

        } else {
        	System.out.println("IMPOSSIBLE");
        }



	}



} public class Main {
	public static void main(String[] args) {


        // Scannerクラスのインスタンスの生成
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        int b = sc.nextInt();
        int k = (a + b) / 2;

        if ((a - k) == -(b - k) || -(a - k) == (b - k)) {
        	System.out.println(k);

        } else {
        	System.out.println("IMPOSSIBLE");
        }



	}



} Main
import java.util.Scanner;
public class Main {
	public static void main(String[] args) {


        // Scannerクラスのインスタンスの生成
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        int b = sc.nextInt();
        int k = (a + b) / 2;

        if ((a - k) == -(b - k) || -(a - k) == (b - k)) {
        	System.out.println(k);

        } else {
        	System.out.println("IMPOSSIBLE");
        }



	}



}