import java.util.Scanner;

public class Main {

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

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

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

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

} public class Main {

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

} Main
import java.util.Scanner;

public class Main {

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

}
