import java.util.Scanner;
public class Main {

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

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

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

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

} public class Main {

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

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

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

}
