import java.util.Scanner;

public class Main {

	public static void main(String[] args) {

		Scanner sc = new Scanner(System.in);

		int sum = sc.nextInt() + sc.nextInt();

		System.out.println(sum % 2 == 0 ? sum / 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 sum = sc.nextInt() + sc.nextInt();

		System.out.println(sum % 2 == 0 ? sum / 2 : "IMPOSSIBLE");
	}
} Main public static void main(String[] args) {

		Scanner sc = new Scanner(System.in);

		int sum = sc.nextInt() + sc.nextInt();

		System.out.println(sum % 2 == 0 ? sum / 2 : "IMPOSSIBLE");
	} main {

		Scanner sc = new Scanner(System.in);

		int sum = sc.nextInt() + sc.nextInt();

		System.out.println(sum % 2 == 0 ? sum / 2 : "IMPOSSIBLE");
	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int sum = sc.nextInt() + sc.nextInt(); sum sc.nextInt() + sc.nextInt() sc.nextInt() sc.nextInt sc sc.nextInt() sc.nextInt sc System.out.println(sum % 2 == 0 ? sum / 2 : "IMPOSSIBLE") System.out.println System.out System System.out sum % 2 == 0 ? sum / 2 : "IMPOSSIBLE" sum % 2 == 0 sum % 2 sum 2 0 sum / 2 sum 2 "IMPOSSIBLE" String[] args args public class Main {

	public static void main(String[] args) {

		Scanner sc = new Scanner(System.in);

		int sum = sc.nextInt() + sc.nextInt();

		System.out.println(sum % 2 == 0 ? sum / 2 : "IMPOSSIBLE");
	}
} public class Main {

	public static void main(String[] args) {

		Scanner sc = new Scanner(System.in);

		int sum = sc.nextInt() + sc.nextInt();

		System.out.println(sum % 2 == 0 ? sum / 2 : "IMPOSSIBLE");
	}
} Main
import java.util.Scanner;

public class Main {

	public static void main(String[] args) {

		Scanner sc = new Scanner(System.in);

		int sum = sc.nextInt() + sc.nextInt();

		System.out.println(sum % 2 == 0 ? sum / 2 : "IMPOSSIBLE");
	}
}