import java.util.Scanner;

public class Main {

	public static void main(String[] args) {

		Scanner scan = new Scanner(System.in);
		int N = scan.nextInt();
		int D = scan.nextInt();
		int sum = N + D;
		if(sum %2 !=0) {
			System.out.println("IMPOSSIBLE");
		}
		else {
			System.out.println(sum/2);
		}
	}

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

	public static void main(String[] args) {

		Scanner scan = new Scanner(System.in);
		int N = scan.nextInt();
		int D = scan.nextInt();
		int sum = N + D;
		if(sum %2 !=0) {
			System.out.println("IMPOSSIBLE");
		}
		else {
			System.out.println(sum/2);
		}
	}

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

		Scanner scan = new Scanner(System.in);
		int N = scan.nextInt();
		int D = scan.nextInt();
		int sum = N + D;
		if(sum %2 !=0) {
			System.out.println("IMPOSSIBLE");
		}
		else {
			System.out.println(sum/2);
		}
	} main {

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

	public static void main(String[] args) {

		Scanner scan = new Scanner(System.in);
		int N = scan.nextInt();
		int D = scan.nextInt();
		int sum = N + D;
		if(sum %2 !=0) {
			System.out.println("IMPOSSIBLE");
		}
		else {
			System.out.println(sum/2);
		}
	}

} public class Main {

	public static void main(String[] args) {

		Scanner scan = new Scanner(System.in);
		int N = scan.nextInt();
		int D = scan.nextInt();
		int sum = N + D;
		if(sum %2 !=0) {
			System.out.println("IMPOSSIBLE");
		}
		else {
			System.out.println(sum/2);
		}
	}

} Main
import java.util.Scanner;

public class Main {

	public static void main(String[] args) {

		Scanner scan = new Scanner(System.in);
		int N = scan.nextInt();
		int D = scan.nextInt();
		int sum = N + D;
		if(sum %2 !=0) {
			System.out.println("IMPOSSIBLE");
		}
		else {
			System.out.println(sum/2);
		}
	}

}