import java.io.FileNotFoundException;
import java.util.Scanner;

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

		//    	File file = new File("src/in.txt");
		//    	Scanner sc = new Scanner(file);

		Scanner sc = new Scanner(System.in);

		int A = sc.nextInt();
		int B = sc.nextInt();
		sc.close();

		if((A+B)%2==0) {
			System.out.println((A+B)/2);
		} else {
			System.out.println("IMPOSSIBLE");
		}
	}
} import java.io.FileNotFoundException; FileNotFoundException java.io import java.util.Scanner; Scanner java.util public class Main {
	public static void main(String[] args) throws FileNotFoundException {

		//    	File file = new File("src/in.txt");
		//    	Scanner sc = new Scanner(file);

		Scanner sc = new Scanner(System.in);

		int A = sc.nextInt();
		int B = sc.nextInt();
		sc.close();

		if((A+B)%2==0) {
			System.out.println((A+B)/2);
		} else {
			System.out.println("IMPOSSIBLE");
		}
	}
} Main public static void main(String[] args) throws FileNotFoundException {

		//    	File file = new File("src/in.txt");
		//    	Scanner sc = new Scanner(file);

		Scanner sc = new Scanner(System.in);

		int A = sc.nextInt();
		int B = sc.nextInt();
		sc.close();

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

		//    	File file = new File("src/in.txt");
		//    	Scanner sc = new Scanner(file);

		Scanner sc = new Scanner(System.in);

		int A = sc.nextInt();
		int B = sc.nextInt();
		sc.close();

		if((A+B)%2==0) {
			System.out.println((A+B)/2);
		} 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 sc.close() sc.close sc if((A+B)%2==0) {
			System.out.println((A+B)/2);
		} else {
			System.out.println("IMPOSSIBLE");
		} (A+B)%2==0 (A+B)%2 (A+B) A B 2 0 {
			System.out.println((A+B)/2);
		} System.out.println((A+B)/2) System.out.println System.out System System.out (A+B)/2 (A+B) A B 2 {
			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) throws FileNotFoundException {

		//    	File file = new File("src/in.txt");
		//    	Scanner sc = new Scanner(file);

		Scanner sc = new Scanner(System.in);

		int A = sc.nextInt();
		int B = sc.nextInt();
		sc.close();

		if((A+B)%2==0) {
			System.out.println((A+B)/2);
		} else {
			System.out.println("IMPOSSIBLE");
		}
	}
} public class Main {
	public static void main(String[] args) throws FileNotFoundException {

		//    	File file = new File("src/in.txt");
		//    	Scanner sc = new Scanner(file);

		Scanner sc = new Scanner(System.in);

		int A = sc.nextInt();
		int B = sc.nextInt();
		sc.close();

		if((A+B)%2==0) {
			System.out.println((A+B)/2);
		} else {
			System.out.println("IMPOSSIBLE");
		}
	}
} Main
import java.io.FileNotFoundException;
import java.util.Scanner;

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

		//    	File file = new File("src/in.txt");
		//    	Scanner sc = new Scanner(file);

		Scanner sc = new Scanner(System.in);

		int A = sc.nextInt();
		int B = sc.nextInt();
		sc.close();

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