import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		String s = sc.nextLine();
		String[] spl = s.split(" ");
		int A = Integer.parseInt(spl[0]);
		int B = Integer.parseInt(spl[1]);
		sc.close();

		if(A == B) {
			System.out.println(0);
		}else 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 sc = new Scanner(System.in);
		String s = sc.nextLine();
		String[] spl = s.split(" ");
		int A = Integer.parseInt(spl[0]);
		int B = Integer.parseInt(spl[1]);
		sc.close();

		if(A == B) {
			System.out.println(0);
		}else if((A + B) % 2 == 0){
			System.out.println((A + B) / 2);
		}else {
			System.out.println("IMPOSSIBLE");
		}
	}
} Main public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		String s = sc.nextLine();
		String[] spl = s.split(" ");
		int A = Integer.parseInt(spl[0]);
		int B = Integer.parseInt(spl[1]);
		sc.close();

		if(A == B) {
			System.out.println(0);
		}else if((A + B) % 2 == 0){
			System.out.println((A + B) / 2);
		}else {
			System.out.println("IMPOSSIBLE");
		}
	} main {
		Scanner sc = new Scanner(System.in);
		String s = sc.nextLine();
		String[] spl = s.split(" ");
		int A = Integer.parseInt(spl[0]);
		int B = Integer.parseInt(spl[1]);
		sc.close();

		if(A == B) {
			System.out.println(0);
		}else if((A + B) % 2 == 0){
			System.out.println((A + B) / 2);
		}else {
			System.out.println("IMPOSSIBLE");
		}
	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) String s = sc.nextLine(); s sc.nextLine() sc.nextLine sc String[] spl = s.split(" "); spl s.split(" ") s.split s " " int A = Integer.parseInt(spl[0]); A Integer.parseInt(spl[0]) Integer.parseInt Integer spl[0] spl 0 int B = Integer.parseInt(spl[1]); B Integer.parseInt(spl[1]) Integer.parseInt Integer spl[1] spl 1 sc.close() sc.close sc if(A == B) {
			System.out.println(0);
		}else if((A + B) % 2 == 0){
			System.out.println((A + B) / 2);
		}else {
			System.out.println("IMPOSSIBLE");
		} A == B A B {
			System.out.println(0);
		} System.out.println(0) System.out.println System.out System System.out 0 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" String[] args args public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		String s = sc.nextLine();
		String[] spl = s.split(" ");
		int A = Integer.parseInt(spl[0]);
		int B = Integer.parseInt(spl[1]);
		sc.close();

		if(A == B) {
			System.out.println(0);
		}else 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 sc = new Scanner(System.in);
		String s = sc.nextLine();
		String[] spl = s.split(" ");
		int A = Integer.parseInt(spl[0]);
		int B = Integer.parseInt(spl[1]);
		sc.close();

		if(A == B) {
			System.out.println(0);
		}else 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 sc = new Scanner(System.in);
		String s = sc.nextLine();
		String[] spl = s.split(" ");
		int A = Integer.parseInt(spl[0]);
		int B = Integer.parseInt(spl[1]);
		sc.close();

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