
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();

		int dis = Math.abs(A-B);
		if (dis%2==0) {
			System.out.println(Math.abs(Math.min(A, B)+dis/2));
		} 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);
		int A = sc.nextInt();
		int B = sc.nextInt();

		int dis = Math.abs(A-B);
		if (dis%2==0) {
			System.out.println(Math.abs(Math.min(A, B)+dis/2));
		} else {
			System.out.println("IMPOSSIBLE");
		}
	}
} Main public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int A = sc.nextInt();
		int B = sc.nextInt();

		int dis = Math.abs(A-B);
		if (dis%2==0) {
			System.out.println(Math.abs(Math.min(A, B)+dis/2));
		} else {
			System.out.println("IMPOSSIBLE");
		}
	} main {
		Scanner sc = new Scanner(System.in);
		int A = sc.nextInt();
		int B = sc.nextInt();

		int dis = Math.abs(A-B);
		if (dis%2==0) {
			System.out.println(Math.abs(Math.min(A, B)+dis/2));
		} else {
			System.out.println("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 int dis = Math.abs(A-B); dis Math.abs(A-B) Math.abs Math A-B A B if (dis%2==0) {
			System.out.println(Math.abs(Math.min(A, B)+dis/2));
		} else {
			System.out.println("IMPOSSIBLE");
		} dis%2==0 dis%2 dis 2 0 {
			System.out.println(Math.abs(Math.min(A, B)+dis/2));
		} System.out.println(Math.abs(Math.min(A, B)+dis/2)) System.out.println System.out System System.out Math.abs(Math.min(A, B)+dis/2) Math.abs Math Math.min(A, B)+dis/2 Math.min(A, B) Math.min Math A B dis/2 dis 2 {
			System.out.println("IMPOSSIBLE");
		} 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);
		int A = sc.nextInt();
		int B = sc.nextInt();

		int dis = Math.abs(A-B);
		if (dis%2==0) {
			System.out.println(Math.abs(Math.min(A, B)+dis/2));
		} else {
			System.out.println("IMPOSSIBLE");
		}
	}
} public class Main {

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

		int dis = Math.abs(A-B);
		if (dis%2==0) {
			System.out.println(Math.abs(Math.min(A, B)+dis/2));
		} 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);
		int A = sc.nextInt();
		int B = sc.nextInt();

		int dis = Math.abs(A-B);
		if (dis%2==0) {
			System.out.println(Math.abs(Math.min(A, B)+dis/2));
		} else {
			System.out.println("IMPOSSIBLE");
		}
	}
}


