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();

		System.out.print((A + B) % 2 == 0 ? (A + B) / 2 : "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 = sc.nextInt();
		int B = sc.nextInt();

		System.out.print((A + B) % 2 == 0 ? (A + B) / 2 : "IMPOSSIBLE");

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

		Scanner sc = new Scanner(System.in);

		int A = sc.nextInt();
		int B = sc.nextInt();

		System.out.print((A + B) % 2 == 0 ? (A + B) / 2 : "IMPOSSIBLE");

	} main {

		Scanner sc = new Scanner(System.in);

		int A = sc.nextInt();
		int B = sc.nextInt();

		System.out.print((A + B) % 2 == 0 ? (A + B) / 2 : "IMPOSSIBLE");

	} 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 System.out.print((A + B) % 2 == 0 ? (A + B) / 2 : "IMPOSSIBLE") System.out.print System.out System System.out (A + B) % 2 == 0 ? (A + B) / 2 : "IMPOSSIBLE" (A + B) % 2 == 0 (A + B) % 2 (A + B) A B 2 0 (A + B) / 2 (A + B) A B 2 "IMPOSSIBLE" 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();

		System.out.print((A + B) % 2 == 0 ? (A + B) / 2 : "IMPOSSIBLE");

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

		Scanner sc = new Scanner(System.in);

		int A = sc.nextInt();
		int B = sc.nextInt();

		System.out.print((A + B) % 2 == 0 ? (A + B) / 2 : "IMPOSSIBLE");

	}
} 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();

		System.out.print((A + B) % 2 == 0 ? (A + B) / 2 : "IMPOSSIBLE");

	}
}