
import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		Scanner scan=new Scanner(System.in);
		int A=scan.nextInt();
		int B=scan.nextInt();
		if(A%2!=B%2) {
			System.out.println("IMPOSSIBLE");
		}
		else {
			System.out.println((A+B)/2);
		}

	}

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

	public static void main(String[] args) {
		Scanner scan=new Scanner(System.in);
		int A=scan.nextInt();
		int B=scan.nextInt();
		if(A%2!=B%2) {
			System.out.println("IMPOSSIBLE");
		}
		else {
			System.out.println((A+B)/2);
		}

	}

} Main public static void main(String[] args) {
		Scanner scan=new Scanner(System.in);
		int A=scan.nextInt();
		int B=scan.nextInt();
		if(A%2!=B%2) {
			System.out.println("IMPOSSIBLE");
		}
		else {
			System.out.println((A+B)/2);
		}

	} main {
		Scanner scan=new Scanner(System.in);
		int A=scan.nextInt();
		int B=scan.nextInt();
		if(A%2!=B%2) {
			System.out.println("IMPOSSIBLE");
		}
		else {
			System.out.println((A+B)/2);
		}

	} Scanner scan=new Scanner(System.in); scan new Scanner(System.in) int A=scan.nextInt(); A scan.nextInt() scan.nextInt scan int B=scan.nextInt(); B scan.nextInt() scan.nextInt scan if(A%2!=B%2) {
			System.out.println("IMPOSSIBLE");
		}
		else {
			System.out.println((A+B)/2);
		} A%2!=B%2 A%2 A 2 B%2 B 2 {
			System.out.println("IMPOSSIBLE");
		} System.out.println("IMPOSSIBLE") System.out.println System.out System System.out "IMPOSSIBLE" {
			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 String[] args args public class Main {

	public static void main(String[] args) {
		Scanner scan=new Scanner(System.in);
		int A=scan.nextInt();
		int B=scan.nextInt();
		if(A%2!=B%2) {
			System.out.println("IMPOSSIBLE");
		}
		else {
			System.out.println((A+B)/2);
		}

	}

} public class Main {

	public static void main(String[] args) {
		Scanner scan=new Scanner(System.in);
		int A=scan.nextInt();
		int B=scan.nextInt();
		if(A%2!=B%2) {
			System.out.println("IMPOSSIBLE");
		}
		else {
			System.out.println((A+B)/2);
		}

	}

} Main

import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		Scanner scan=new Scanner(System.in);
		int A=scan.nextInt();
		int B=scan.nextInt();
		if(A%2!=B%2) {
			System.out.println("IMPOSSIBLE");
		}
		else {
			System.out.println((A+B)/2);
		}

	}

}