import java.util.Scanner;
public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		long A = sc.nextLong();
		long B = sc.nextLong();
		if((A+B)%2 != 0)System.out.println("IMPOSSIBLE");
		else {
			long AB = (A+B)/2;
			System.out.println(Math.abs(AB));
		}
	}
} import java.util.Scanner; Scanner java.util public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		long A = sc.nextLong();
		long B = sc.nextLong();
		if((A+B)%2 != 0)System.out.println("IMPOSSIBLE");
		else {
			long AB = (A+B)/2;
			System.out.println(Math.abs(AB));
		}
	}
} Main public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		long A = sc.nextLong();
		long B = sc.nextLong();
		if((A+B)%2 != 0)System.out.println("IMPOSSIBLE");
		else {
			long AB = (A+B)/2;
			System.out.println(Math.abs(AB));
		}
	} main {
		Scanner sc = new Scanner(System.in);
		long A = sc.nextLong();
		long B = sc.nextLong();
		if((A+B)%2 != 0)System.out.println("IMPOSSIBLE");
		else {
			long AB = (A+B)/2;
			System.out.println(Math.abs(AB));
		}
	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) long A = sc.nextLong(); A sc.nextLong() sc.nextLong sc long B = sc.nextLong(); B sc.nextLong() sc.nextLong sc if((A+B)%2 != 0)System.out.println("IMPOSSIBLE");
		else {
			long AB = (A+B)/2;
			System.out.println(Math.abs(AB));
		} (A+B)%2 != 0 (A+B)%2 (A+B) A B 2 0 System.out.println("IMPOSSIBLE") System.out.println System.out System System.out "IMPOSSIBLE" {
			long AB = (A+B)/2;
			System.out.println(Math.abs(AB));
		} long AB = (A+B)/2; AB (A+B)/2 (A+B) A B 2 System.out.println(Math.abs(AB)) System.out.println System.out System System.out Math.abs(AB) Math.abs Math AB String[] args args public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		long A = sc.nextLong();
		long B = sc.nextLong();
		if((A+B)%2 != 0)System.out.println("IMPOSSIBLE");
		else {
			long AB = (A+B)/2;
			System.out.println(Math.abs(AB));
		}
	}
} public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		long A = sc.nextLong();
		long B = sc.nextLong();
		if((A+B)%2 != 0)System.out.println("IMPOSSIBLE");
		else {
			long AB = (A+B)/2;
			System.out.println(Math.abs(AB));
		}
	}
} Main
import java.util.Scanner;
public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		long A = sc.nextLong();
		long B = sc.nextLong();
		if((A+B)%2 != 0)System.out.println("IMPOSSIBLE");
		else {
			long AB = (A+B)/2;
			System.out.println(Math.abs(AB));
		}
	}
}