import java.util.Scanner;
public class Main {

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

	}

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

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

	}

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

	} main {
		Scanner input =new Scanner(System.in);
		long a=input.nextLong();
		long b=input.nextLong();
		input.close();
		if((a+b)%2==0)
		{
			System.out.println((a+b)/2);
		}
		else
		{
			System.out.println("IMPOSSIBLE");
		}
		System.exit(0);
	

	} Scanner input =new Scanner(System.in); input new Scanner(System.in) long a=input.nextLong(); a input.nextLong() input.nextLong input long b=input.nextLong(); b input.nextLong() input.nextLong input input.close() input.close input 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" System.exit(0) System.exit System 0 String[] args args public class Main {

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

	}

} public class Main {

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

	}

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

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

	}

}
