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();

		int k = (a+b)/2;
		String ans = "";

		if ((a+b)%2 == 0 && Math.abs(a-k) == Math.abs(b-k)) {

			ans = String.valueOf(k);

		} else {
			ans = "IMPOSSIBLE";
		}

		System.out.println(ans);
		sc.close();

	}

} 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();

		int k = (a+b)/2;
		String ans = "";

		if ((a+b)%2 == 0 && Math.abs(a-k) == Math.abs(b-k)) {

			ans = String.valueOf(k);

		} else {
			ans = "IMPOSSIBLE";
		}

		System.out.println(ans);
		sc.close();

	}

} Main public static void main(String[] args) {

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

		int k = (a+b)/2;
		String ans = "";

		if ((a+b)%2 == 0 && Math.abs(a-k) == Math.abs(b-k)) {

			ans = String.valueOf(k);

		} else {
			ans = "IMPOSSIBLE";
		}

		System.out.println(ans);
		sc.close();

	} main {

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

		int k = (a+b)/2;
		String ans = "";

		if ((a+b)%2 == 0 && Math.abs(a-k) == Math.abs(b-k)) {

			ans = String.valueOf(k);

		} else {
			ans = "IMPOSSIBLE";
		}

		System.out.println(ans);
		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 k = (a+b)/2; k (a+b)/2 (a+b) a b 2 String ans = ""; ans "" if ((a+b)%2 == 0 && Math.abs(a-k) == Math.abs(b-k)) {

			ans = String.valueOf(k);

		} else {
			ans = "IMPOSSIBLE";
		} (a+b)%2 == 0 && Math.abs(a-k) == Math.abs(b-k) (a+b)%2 == 0 (a+b)%2 (a+b) a b 2 0 Math.abs(a-k) == Math.abs(b-k) Math.abs(a-k) Math.abs Math a-k a k Math.abs(b-k) Math.abs Math b-k b k {

			ans = String.valueOf(k);

		} ans = String.valueOf(k) ans String.valueOf(k) String.valueOf String k {
			ans = "IMPOSSIBLE";
		} ans = "IMPOSSIBLE" ans "IMPOSSIBLE" System.out.println(ans) System.out.println System.out System System.out ans sc.close() sc.close sc 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();

		int k = (a+b)/2;
		String ans = "";

		if ((a+b)%2 == 0 && Math.abs(a-k) == Math.abs(b-k)) {

			ans = String.valueOf(k);

		} else {
			ans = "IMPOSSIBLE";
		}

		System.out.println(ans);
		sc.close();

	}

} public class Main {

	public static void main(String[] args) {

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

		int k = (a+b)/2;
		String ans = "";

		if ((a+b)%2 == 0 && Math.abs(a-k) == Math.abs(b-k)) {

			ans = String.valueOf(k);

		} else {
			ans = "IMPOSSIBLE";
		}

		System.out.println(ans);
		sc.close();

	}

} 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();

		int k = (a+b)/2;
		String ans = "";

		if ((a+b)%2 == 0 && Math.abs(a-k) == Math.abs(b-k)) {

			ans = String.valueOf(k);

		} else {
			ans = "IMPOSSIBLE";
		}

		System.out.println(ans);
		sc.close();

	}

}
