import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		Scanner s = new Scanner(System.in);
		long a = s.nextLong();
		long b = s.nextLong();
		s.close();
		long disA = 0;
		long disB = 0;

		if(a < 0) {
			disA = a * -1;
		}else {
			disA = a;
		}
		if(b < 0) {
			disB = b * -1;
		}else {
			disB = b;
		}

		long judge = (disA + disB) % 2;
		if(judge == 1) {
			System.out.println("IMPOSSIBLE");
		}else {
			long disC = (disA + disB)/2;
			System.out.println(disC);
		}

	}

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

	public static void main(String[] args) {
		Scanner s = new Scanner(System.in);
		long a = s.nextLong();
		long b = s.nextLong();
		s.close();
		long disA = 0;
		long disB = 0;

		if(a < 0) {
			disA = a * -1;
		}else {
			disA = a;
		}
		if(b < 0) {
			disB = b * -1;
		}else {
			disB = b;
		}

		long judge = (disA + disB) % 2;
		if(judge == 1) {
			System.out.println("IMPOSSIBLE");
		}else {
			long disC = (disA + disB)/2;
			System.out.println(disC);
		}

	}

} Main public static void main(String[] args) {
		Scanner s = new Scanner(System.in);
		long a = s.nextLong();
		long b = s.nextLong();
		s.close();
		long disA = 0;
		long disB = 0;

		if(a < 0) {
			disA = a * -1;
		}else {
			disA = a;
		}
		if(b < 0) {
			disB = b * -1;
		}else {
			disB = b;
		}

		long judge = (disA + disB) % 2;
		if(judge == 1) {
			System.out.println("IMPOSSIBLE");
		}else {
			long disC = (disA + disB)/2;
			System.out.println(disC);
		}

	} main {
		Scanner s = new Scanner(System.in);
		long a = s.nextLong();
		long b = s.nextLong();
		s.close();
		long disA = 0;
		long disB = 0;

		if(a < 0) {
			disA = a * -1;
		}else {
			disA = a;
		}
		if(b < 0) {
			disB = b * -1;
		}else {
			disB = b;
		}

		long judge = (disA + disB) % 2;
		if(judge == 1) {
			System.out.println("IMPOSSIBLE");
		}else {
			long disC = (disA + disB)/2;
			System.out.println(disC);
		}

	} Scanner s = new Scanner(System.in); s new Scanner(System.in) long a = s.nextLong(); a s.nextLong() s.nextLong s long b = s.nextLong(); b s.nextLong() s.nextLong s s.close() s.close s long disA = 0; disA 0 long disB = 0; disB 0 if(a < 0) {
			disA = a * -1;
		}else {
			disA = a;
		} a < 0 a 0 {
			disA = a * -1;
		} disA = a * -1 disA a * -1 a -1 1 {
			disA = a;
		} disA = a disA a if(b < 0) {
			disB = b * -1;
		}else {
			disB = b;
		} b < 0 b 0 {
			disB = b * -1;
		} disB = b * -1 disB b * -1 b -1 1 {
			disB = b;
		} disB = b disB b long judge = (disA + disB) % 2; judge (disA + disB) % 2 (disA + disB) disA disB 2 if(judge == 1) {
			System.out.println("IMPOSSIBLE");
		}else {
			long disC = (disA + disB)/2;
			System.out.println(disC);
		} judge == 1 judge 1 {
			System.out.println("IMPOSSIBLE");
		} System.out.println("IMPOSSIBLE") System.out.println System.out System System.out "IMPOSSIBLE" {
			long disC = (disA + disB)/2;
			System.out.println(disC);
		} long disC = (disA + disB)/2; disC (disA + disB)/2 (disA + disB) disA disB 2 System.out.println(disC) System.out.println System.out System System.out disC String[] args args public class Main {

	public static void main(String[] args) {
		Scanner s = new Scanner(System.in);
		long a = s.nextLong();
		long b = s.nextLong();
		s.close();
		long disA = 0;
		long disB = 0;

		if(a < 0) {
			disA = a * -1;
		}else {
			disA = a;
		}
		if(b < 0) {
			disB = b * -1;
		}else {
			disB = b;
		}

		long judge = (disA + disB) % 2;
		if(judge == 1) {
			System.out.println("IMPOSSIBLE");
		}else {
			long disC = (disA + disB)/2;
			System.out.println(disC);
		}

	}

} public class Main {

	public static void main(String[] args) {
		Scanner s = new Scanner(System.in);
		long a = s.nextLong();
		long b = s.nextLong();
		s.close();
		long disA = 0;
		long disB = 0;

		if(a < 0) {
			disA = a * -1;
		}else {
			disA = a;
		}
		if(b < 0) {
			disB = b * -1;
		}else {
			disB = b;
		}

		long judge = (disA + disB) % 2;
		if(judge == 1) {
			System.out.println("IMPOSSIBLE");
		}else {
			long disC = (disA + disB)/2;
			System.out.println(disC);
		}

	}

} Main
import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		Scanner s = new Scanner(System.in);
		long a = s.nextLong();
		long b = s.nextLong();
		s.close();
		long disA = 0;
		long disB = 0;

		if(a < 0) {
			disA = a * -1;
		}else {
			disA = a;
		}
		if(b < 0) {
			disB = b * -1;
		}else {
			disB = b;
		}

		long judge = (disA + disB) % 2;
		if(judge == 1) {
			System.out.println("IMPOSSIBLE");
		}else {
			long disC = (disA + disB)/2;
			System.out.println(disC);
		}

	}

}
