import java.util.Scanner;

public class Main {

	public static void main(String[] args) {

		Scanner		scan	=	new Scanner(System.in);
		int a = scan.nextInt();
		int b = scan.nextInt();



		scan.close();

		if((a + b) % 2 == 0) {
			int k = (a + b) / 2;
			System.out.println(k);
		} else {
			System.out.println("IMPOSSIBLE");
		}

		scan.close();

	}


} import java.util.Scanner; Scanner java.util public class Main {

	public static void main(String[] args) {

		Scanner		scan	=	new Scanner(System.in);
		int a = scan.nextInt();
		int b = scan.nextInt();



		scan.close();

		if((a + b) % 2 == 0) {
			int k = (a + b) / 2;
			System.out.println(k);
		} else {
			System.out.println("IMPOSSIBLE");
		}

		scan.close();

	}


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

		Scanner		scan	=	new Scanner(System.in);
		int a = scan.nextInt();
		int b = scan.nextInt();



		scan.close();

		if((a + b) % 2 == 0) {
			int k = (a + b) / 2;
			System.out.println(k);
		} else {
			System.out.println("IMPOSSIBLE");
		}

		scan.close();

	} main {

		Scanner		scan	=	new Scanner(System.in);
		int a = scan.nextInt();
		int b = scan.nextInt();



		scan.close();

		if((a + b) % 2 == 0) {
			int k = (a + b) / 2;
			System.out.println(k);
		} else {
			System.out.println("IMPOSSIBLE");
		}

		scan.close();

	} Scanner		scan	=	new Scanner(System.in); scan new Scanner(System.in) int a = scan.nextInt(); a scan.nextInt() scan.nextInt scan int b = scan.nextInt(); b scan.nextInt() scan.nextInt scan scan.close() scan.close scan if((a + b) % 2 == 0) {
			int k = (a + b) / 2;
			System.out.println(k);
		} else {
			System.out.println("IMPOSSIBLE");
		} (a + b) % 2 == 0 (a + b) % 2 (a + b) a b 2 0 {
			int k = (a + b) / 2;
			System.out.println(k);
		} int k = (a + b) / 2; k (a + b) / 2 (a + b) a b 2 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" scan.close() scan.close scan String[] args args public class Main {

	public static void main(String[] args) {

		Scanner		scan	=	new Scanner(System.in);
		int a = scan.nextInt();
		int b = scan.nextInt();



		scan.close();

		if((a + b) % 2 == 0) {
			int k = (a + b) / 2;
			System.out.println(k);
		} else {
			System.out.println("IMPOSSIBLE");
		}

		scan.close();

	}


} public class Main {

	public static void main(String[] args) {

		Scanner		scan	=	new Scanner(System.in);
		int a = scan.nextInt();
		int b = scan.nextInt();



		scan.close();

		if((a + b) % 2 == 0) {
			int k = (a + b) / 2;
			System.out.println(k);
		} else {
			System.out.println("IMPOSSIBLE");
		}

		scan.close();

	}


} Main
import java.util.Scanner;

public class Main {

	public static void main(String[] args) {

		Scanner		scan	=	new Scanner(System.in);
		int a = scan.nextInt();
		int b = scan.nextInt();



		scan.close();

		if((a + b) % 2 == 0) {
			int k = (a + b) / 2;
			System.out.println(k);
		} else {
			System.out.println("IMPOSSIBLE");
		}

		scan.close();

	}


}