import java.util.*;
public class Main {

	public static void main(String[] args) {
		Scanner stdIn = new Scanner(System.in);
		int A = stdIn.nextInt();
		int B = stdIn.nextInt();
		
		if((Math.max(A,B)-Math.min(A,B))%2 == 0) {
			System.out.println((Math.max(A,B)+Math.min(A,B))/2);
		}
		else {
			System.out.println("IMPOSSIBLE");
		}

	}

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

	public static void main(String[] args) {
		Scanner stdIn = new Scanner(System.in);
		int A = stdIn.nextInt();
		int B = stdIn.nextInt();
		
		if((Math.max(A,B)-Math.min(A,B))%2 == 0) {
			System.out.println((Math.max(A,B)+Math.min(A,B))/2);
		}
		else {
			System.out.println("IMPOSSIBLE");
		}

	}

} Main public static void main(String[] args) {
		Scanner stdIn = new Scanner(System.in);
		int A = stdIn.nextInt();
		int B = stdIn.nextInt();
		
		if((Math.max(A,B)-Math.min(A,B))%2 == 0) {
			System.out.println((Math.max(A,B)+Math.min(A,B))/2);
		}
		else {
			System.out.println("IMPOSSIBLE");
		}

	} main {
		Scanner stdIn = new Scanner(System.in);
		int A = stdIn.nextInt();
		int B = stdIn.nextInt();
		
		if((Math.max(A,B)-Math.min(A,B))%2 == 0) {
			System.out.println((Math.max(A,B)+Math.min(A,B))/2);
		}
		else {
			System.out.println("IMPOSSIBLE");
		}

	} Scanner stdIn = new Scanner(System.in); stdIn new Scanner(System.in) int A = stdIn.nextInt(); A stdIn.nextInt() stdIn.nextInt stdIn int B = stdIn.nextInt(); B stdIn.nextInt() stdIn.nextInt stdIn if((Math.max(A,B)-Math.min(A,B))%2 == 0) {
			System.out.println((Math.max(A,B)+Math.min(A,B))/2);
		}
		else {
			System.out.println("IMPOSSIBLE");
		} (Math.max(A,B)-Math.min(A,B))%2 == 0 (Math.max(A,B)-Math.min(A,B))%2 (Math.max(A,B)-Math.min(A,B)) Math.max(A,B) Math.max Math A B Math.min(A,B) Math.min Math A B 2 0 {
			System.out.println((Math.max(A,B)+Math.min(A,B))/2);
		} System.out.println((Math.max(A,B)+Math.min(A,B))/2) System.out.println System.out System System.out (Math.max(A,B)+Math.min(A,B))/2 (Math.max(A,B)+Math.min(A,B)) Math.max(A,B) Math.max Math A B Math.min(A,B) Math.min Math 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) {
		Scanner stdIn = new Scanner(System.in);
		int A = stdIn.nextInt();
		int B = stdIn.nextInt();
		
		if((Math.max(A,B)-Math.min(A,B))%2 == 0) {
			System.out.println((Math.max(A,B)+Math.min(A,B))/2);
		}
		else {
			System.out.println("IMPOSSIBLE");
		}

	}

} public class Main {

	public static void main(String[] args) {
		Scanner stdIn = new Scanner(System.in);
		int A = stdIn.nextInt();
		int B = stdIn.nextInt();
		
		if((Math.max(A,B)-Math.min(A,B))%2 == 0) {
			System.out.println((Math.max(A,B)+Math.min(A,B))/2);
		}
		else {
			System.out.println("IMPOSSIBLE");
		}

	}

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

	public static void main(String[] args) {
		Scanner stdIn = new Scanner(System.in);
		int A = stdIn.nextInt();
		int B = stdIn.nextInt();
		
		if((Math.max(A,B)-Math.min(A,B))%2 == 0) {
			System.out.println((Math.max(A,B)+Math.min(A,B))/2);
		}
		else {
			System.out.println("IMPOSSIBLE");
		}

	}

}