import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int a = sc.nextInt();
		int b = sc.nextInt();

		sc.close();

		int tmp = 1;
		tmp = a + b;
		if(tmp%2==0) {
			System.out.println(tmp/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);
		int a = sc.nextInt();
		int b = sc.nextInt();

		sc.close();

		int tmp = 1;
		tmp = a + b;
		if(tmp%2==0) {
			System.out.println(tmp/2);
		}else {
			System.out.println("IMPOSSIBLE");
		}


	}
} Main public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int a = sc.nextInt();
		int b = sc.nextInt();

		sc.close();

		int tmp = 1;
		tmp = a + b;
		if(tmp%2==0) {
			System.out.println(tmp/2);
		}else {
			System.out.println("IMPOSSIBLE");
		}


	} main {
		Scanner sc = new Scanner(System.in);
		int a = sc.nextInt();
		int b = sc.nextInt();

		sc.close();

		int tmp = 1;
		tmp = a + b;
		if(tmp%2==0) {
			System.out.println(tmp/2);
		}else {
			System.out.println("IMPOSSIBLE");
		}


	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int a = sc.nextInt(); a sc.nextInt() sc.nextInt sc int b = sc.nextInt(); b sc.nextInt() sc.nextInt sc sc.close() sc.close sc int tmp = 1; tmp 1 tmp = a + b tmp a + b a b if(tmp%2==0) {
			System.out.println(tmp/2);
		}else {
			System.out.println("IMPOSSIBLE");
		} tmp%2==0 tmp%2 tmp 2 0 {
			System.out.println(tmp/2);
		} System.out.println(tmp/2) System.out.println System.out System System.out tmp/2 tmp 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);
		int a = sc.nextInt();
		int b = sc.nextInt();

		sc.close();

		int tmp = 1;
		tmp = a + b;
		if(tmp%2==0) {
			System.out.println(tmp/2);
		}else {
			System.out.println("IMPOSSIBLE");
		}


	}
} public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int a = sc.nextInt();
		int b = sc.nextInt();

		sc.close();

		int tmp = 1;
		tmp = a + b;
		if(tmp%2==0) {
			System.out.println(tmp/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);
		int a = sc.nextInt();
		int b = sc.nextInt();

		sc.close();

		int tmp = 1;
		tmp = a + b;
		if(tmp%2==0) {
			System.out.println(tmp/2);
		}else {
			System.out.println("IMPOSSIBLE");
		}


	}
}