import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		double A = sc.nextInt();
		double B = sc.nextInt();
		double K = (A + B) / 2;

		if (K == (int) K) System.out.println((int) K);
		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);
		double A = sc.nextInt();
		double B = sc.nextInt();
		double K = (A + B) / 2;

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

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

		if (K == (int) K) System.out.println((int) K);
		else System.out.println("IMPOSSIBLE");
	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) double A = sc.nextInt(); A sc.nextInt() sc.nextInt sc double B = sc.nextInt(); B sc.nextInt() sc.nextInt sc double K = (A + B) / 2; K (A + B) / 2 (A + B) A B 2 if (K == (int) K) System.out.println((int) K);
		else System.out.println("IMPOSSIBLE"); K == (int) K K (int) K System.out.println((int) K) System.out.println System.out System System.out (int) K 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);
		double A = sc.nextInt();
		double B = sc.nextInt();
		double K = (A + B) / 2;

		if (K == (int) K) System.out.println((int) K);
		else System.out.println("IMPOSSIBLE");
	}
} public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		double A = sc.nextInt();
		double B = sc.nextInt();
		double K = (A + B) / 2;

		if (K == (int) K) System.out.println((int) K);
		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);
		double A = sc.nextInt();
		double B = sc.nextInt();
		double K = (A + B) / 2;

		if (K == (int) K) System.out.println((int) K);
		else System.out.println("IMPOSSIBLE");
	}
}