import java.util.Scanner;


public class Main{

	public static void main(String[] args) {
		Scanner input = new Scanner(System.in);
		int A = input.nextInt();
		int B = input.nextInt();
		if ((A + B) % 2 ==0) {
			System.out.print((A+B)/2);

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

	}

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

	public static void main(String[] args) {
		Scanner input = new Scanner(System.in);
		int A = input.nextInt();
		int B = input.nextInt();
		if ((A + B) % 2 ==0) {
			System.out.print((A+B)/2);

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

	}

} Main public static void main(String[] args) {
		Scanner input = new Scanner(System.in);
		int A = input.nextInt();
		int B = input.nextInt();
		if ((A + B) % 2 ==0) {
			System.out.print((A+B)/2);

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

	} main {
		Scanner input = new Scanner(System.in);
		int A = input.nextInt();
		int B = input.nextInt();
		if ((A + B) % 2 ==0) {
			System.out.print((A+B)/2);

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

	} Scanner input = new Scanner(System.in); input new Scanner(System.in) int A = input.nextInt(); A input.nextInt() input.nextInt input int B = input.nextInt(); B input.nextInt() input.nextInt input if ((A + B) % 2 ==0) {
			System.out.print((A+B)/2);

		}else {
			System.out.print("IMPOSSIBLE");
		} (A + B) % 2 ==0 (A + B) % 2 (A + B) A B 2 0 {
			System.out.print((A+B)/2);

		} System.out.print((A+B)/2) System.out.print System.out System System.out (A+B)/2 (A+B) A B 2 {
			System.out.print("IMPOSSIBLE");
		} System.out.print("IMPOSSIBLE") System.out.print System.out System System.out "IMPOSSIBLE" String[] args args public class Main{

	public static void main(String[] args) {
		Scanner input = new Scanner(System.in);
		int A = input.nextInt();
		int B = input.nextInt();
		if ((A + B) % 2 ==0) {
			System.out.print((A+B)/2);

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

	}

} public class Main{

	public static void main(String[] args) {
		Scanner input = new Scanner(System.in);
		int A = input.nextInt();
		int B = input.nextInt();
		if ((A + B) % 2 ==0) {
			System.out.print((A+B)/2);

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

	}

} Main
import java.util.Scanner;


public class Main{

	public static void main(String[] args) {
		Scanner input = new Scanner(System.in);
		int A = input.nextInt();
		int B = input.nextInt();
		if ((A + B) % 2 ==0) {
			System.out.print((A+B)/2);

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

	}

}