import java.util.*;

public class Main {
	public static void main(String[] args) {
		Main main = new Main();
		main.run();
	}

	public void run() {
		Scanner sc = new Scanner(System.in);
		int a=sc.nextInt();
		int b=sc.nextInt();
		int ab = a+b;
		if(ab%2==1) {
			System.out.println("IMPOSSIBLE");
		}else {
			System.out.println(ab/2);
		}
		sc.close();
	}
} import java.util.*; util.* java public class Main {
	public static void main(String[] args) {
		Main main = new Main();
		main.run();
	}

	public void run() {
		Scanner sc = new Scanner(System.in);
		int a=sc.nextInt();
		int b=sc.nextInt();
		int ab = a+b;
		if(ab%2==1) {
			System.out.println("IMPOSSIBLE");
		}else {
			System.out.println(ab/2);
		}
		sc.close();
	}
} Main public static void main(String[] args) {
		Main main = new Main();
		main.run();
	} main {
		Main main = new Main();
		main.run();
	} Main main = new Main(); main new Main() main.run() main.run main String[] args args public void run() {
		Scanner sc = new Scanner(System.in);
		int a=sc.nextInt();
		int b=sc.nextInt();
		int ab = a+b;
		if(ab%2==1) {
			System.out.println("IMPOSSIBLE");
		}else {
			System.out.println(ab/2);
		}
		sc.close();
	} run {
		Scanner sc = new Scanner(System.in);
		int a=sc.nextInt();
		int b=sc.nextInt();
		int ab = a+b;
		if(ab%2==1) {
			System.out.println("IMPOSSIBLE");
		}else {
			System.out.println(ab/2);
		}
		sc.close();
	} 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 int ab = a+b; ab a+b a b if(ab%2==1) {
			System.out.println("IMPOSSIBLE");
		}else {
			System.out.println(ab/2);
		} ab%2==1 ab%2 ab 2 1 {
			System.out.println("IMPOSSIBLE");
		} System.out.println("IMPOSSIBLE") System.out.println System.out System System.out "IMPOSSIBLE" {
			System.out.println(ab/2);
		} System.out.println(ab/2) System.out.println System.out System System.out ab/2 ab 2 sc.close() sc.close sc public class Main {
	public static void main(String[] args) {
		Main main = new Main();
		main.run();
	}

	public void run() {
		Scanner sc = new Scanner(System.in);
		int a=sc.nextInt();
		int b=sc.nextInt();
		int ab = a+b;
		if(ab%2==1) {
			System.out.println("IMPOSSIBLE");
		}else {
			System.out.println(ab/2);
		}
		sc.close();
	}
} public class Main {
	public static void main(String[] args) {
		Main main = new Main();
		main.run();
	}

	public void run() {
		Scanner sc = new Scanner(System.in);
		int a=sc.nextInt();
		int b=sc.nextInt();
		int ab = a+b;
		if(ab%2==1) {
			System.out.println("IMPOSSIBLE");
		}else {
			System.out.println(ab/2);
		}
		sc.close();
	}
} Main
import java.util.*;

public class Main {
	public static void main(String[] args) {
		Main main = new Main();
		main.run();
	}

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