import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		long a = sc.nextLong();
		long b = sc.nextLong();

		long sum = a + b;

		if (sum % 2 != 0) {
			System.out.println("IMPOSSIBLE");
			return;
		}

		System.out.println(sum / 2);
	}
} import java.util.Scanner; Scanner java.util public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		long a = sc.nextLong();
		long b = sc.nextLong();

		long sum = a + b;

		if (sum % 2 != 0) {
			System.out.println("IMPOSSIBLE");
			return;
		}

		System.out.println(sum / 2);
	}
} Main public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		long a = sc.nextLong();
		long b = sc.nextLong();

		long sum = a + b;

		if (sum % 2 != 0) {
			System.out.println("IMPOSSIBLE");
			return;
		}

		System.out.println(sum / 2);
	} main {
		Scanner sc = new Scanner(System.in);
		long a = sc.nextLong();
		long b = sc.nextLong();

		long sum = a + b;

		if (sum % 2 != 0) {
			System.out.println("IMPOSSIBLE");
			return;
		}

		System.out.println(sum / 2);
	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) long a = sc.nextLong(); a sc.nextLong() sc.nextLong sc long b = sc.nextLong(); b sc.nextLong() sc.nextLong sc long sum = a + b; sum a + b a b if (sum % 2 != 0) {
			System.out.println("IMPOSSIBLE");
			return;
		} sum % 2 != 0 sum % 2 sum 2 0 {
			System.out.println("IMPOSSIBLE");
			return;
		} System.out.println("IMPOSSIBLE") System.out.println System.out System System.out "IMPOSSIBLE" return; System.out.println(sum / 2) System.out.println System.out System System.out sum / 2 sum 2 String[] args args public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		long a = sc.nextLong();
		long b = sc.nextLong();

		long sum = a + b;

		if (sum % 2 != 0) {
			System.out.println("IMPOSSIBLE");
			return;
		}

		System.out.println(sum / 2);
	}
} public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		long a = sc.nextLong();
		long b = sc.nextLong();

		long sum = a + b;

		if (sum % 2 != 0) {
			System.out.println("IMPOSSIBLE");
			return;
		}

		System.out.println(sum / 2);
	}
} Main
import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		long a = sc.nextLong();
		long b = sc.nextLong();

		long sum = a + b;

		if (sum % 2 != 0) {
			System.out.println("IMPOSSIBLE");
			return;
		}

		System.out.println(sum / 2);
	}
}