import java.util.Scanner;

public class Main {
	public static void main(String args[]) {
		Scanner scanner = new Scanner(System.in);
		int A = scanner.nextInt();
		int B = scanner.nextInt();
		scanner.close();

		if (((A + B) & 1) == 0)
			System.out.println((A + B) >> 1);
		else
			System.out.println("IMPOSSIBLE");
	}
} import java.util.Scanner; Scanner java.util public class Main {
	public static void main(String args[]) {
		Scanner scanner = new Scanner(System.in);
		int A = scanner.nextInt();
		int B = scanner.nextInt();
		scanner.close();

		if (((A + B) & 1) == 0)
			System.out.println((A + B) >> 1);
		else
			System.out.println("IMPOSSIBLE");
	}
} Main public static void main(String args[]) {
		Scanner scanner = new Scanner(System.in);
		int A = scanner.nextInt();
		int B = scanner.nextInt();
		scanner.close();

		if (((A + B) & 1) == 0)
			System.out.println((A + B) >> 1);
		else
			System.out.println("IMPOSSIBLE");
	} main {
		Scanner scanner = new Scanner(System.in);
		int A = scanner.nextInt();
		int B = scanner.nextInt();
		scanner.close();

		if (((A + B) & 1) == 0)
			System.out.println((A + B) >> 1);
		else
			System.out.println("IMPOSSIBLE");
	} Scanner scanner = new Scanner(System.in); scanner new Scanner(System.in) int A = scanner.nextInt(); A scanner.nextInt() scanner.nextInt scanner int B = scanner.nextInt(); B scanner.nextInt() scanner.nextInt scanner scanner.close() scanner.close scanner if (((A + B) & 1) == 0)
			System.out.println((A + B) >> 1);
		else
			System.out.println("IMPOSSIBLE"); ((A + B) & 1) == 0 ((A + B) & 1) (A + B) A B 1 0 System.out.println((A + B) >> 1) System.out.println System.out System System.out (A + B) >> 1 (A + B) A B 1 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 scanner = new Scanner(System.in);
		int A = scanner.nextInt();
		int B = scanner.nextInt();
		scanner.close();

		if (((A + B) & 1) == 0)
			System.out.println((A + B) >> 1);
		else
			System.out.println("IMPOSSIBLE");
	}
} public class Main {
	public static void main(String args[]) {
		Scanner scanner = new Scanner(System.in);
		int A = scanner.nextInt();
		int B = scanner.nextInt();
		scanner.close();

		if (((A + B) & 1) == 0)
			System.out.println((A + B) >> 1);
		else
			System.out.println("IMPOSSIBLE");
	}
} Main
import java.util.Scanner;

public class Main {
	public static void main(String args[]) {
		Scanner scanner = new Scanner(System.in);
		int A = scanner.nextInt();
		int B = scanner.nextInt();
		scanner.close();

		if (((A + B) & 1) == 0)
			System.out.println((A + B) >> 1);
		else
			System.out.println("IMPOSSIBLE");
	}
}