import java.util.Scanner;

public class Main {
	public static void main(String[] args){
		Scanner stdIn = new Scanner(System.in);
		long A = stdIn.nextLong();
		long B = stdIn.nextLong();

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

		if(((A + B) % 2) == 0){
			System.out.println((A + B) / 2);
		}
		else{
			System.out.println("IMPOSSIBLE");
		}
		stdIn.close();
	}
} Main public static void main(String[] args){
		Scanner stdIn = new Scanner(System.in);
		long A = stdIn.nextLong();
		long B = stdIn.nextLong();

		if(((A + B) % 2) == 0){
			System.out.println((A + B) / 2);
		}
		else{
			System.out.println("IMPOSSIBLE");
		}
		stdIn.close();
	} main {
		Scanner stdIn = new Scanner(System.in);
		long A = stdIn.nextLong();
		long B = stdIn.nextLong();

		if(((A + B) % 2) == 0){
			System.out.println((A + B) / 2);
		}
		else{
			System.out.println("IMPOSSIBLE");
		}
		stdIn.close();
	} Scanner stdIn = new Scanner(System.in); stdIn new Scanner(System.in) long A = stdIn.nextLong(); A stdIn.nextLong() stdIn.nextLong stdIn long B = stdIn.nextLong(); B stdIn.nextLong() stdIn.nextLong stdIn 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" stdIn.close() stdIn.close stdIn String[] args args public class Main {
	public static void main(String[] args){
		Scanner stdIn = new Scanner(System.in);
		long A = stdIn.nextLong();
		long B = stdIn.nextLong();

		if(((A + B) % 2) == 0){
			System.out.println((A + B) / 2);
		}
		else{
			System.out.println("IMPOSSIBLE");
		}
		stdIn.close();
	}
} public class Main {
	public static void main(String[] args){
		Scanner stdIn = new Scanner(System.in);
		long A = stdIn.nextLong();
		long B = stdIn.nextLong();

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

public class Main {
	public static void main(String[] args){
		Scanner stdIn = new Scanner(System.in);
		long A = stdIn.nextLong();
		long B = stdIn.nextLong();

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