
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();

		if((a+b)%2==0) {
			System.out.println((a+b)/2);
		}else if((a+b)%2!=0) {
			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();

		if((a+b)%2==0) {
			System.out.println((a+b)/2);
		}else if((a+b)%2!=0) {
			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();

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

	} main {
		
		Scanner sc=new Scanner(System.in);

		int a=sc.nextInt();
		int b=sc.nextInt();

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

		if((a+b)%2==0) {
			System.out.println((a+b)/2);
		}else if((a+b)%2!=0) {
			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();

		if((a+b)%2==0) {
			System.out.println((a+b)/2);
		}else if((a+b)%2!=0) {
			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();

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

	}

}
