import java.util.Scanner;

public class Main {

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

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