import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		Scanner input = new Scanner(System.in);
		int num1 = input.nextInt();
		int num2 = input.nextInt();
		
		if((num1+num2)%2 == 0)
			System.out.println((num1+num2)/2);
		else
			System.out.println("IMPOSSIBLE");
		
		input.close();

	}

} import java.util.Scanner; Scanner java.util public class Main {

	public static void main(String[] args) {
		Scanner input = new Scanner(System.in);
		int num1 = input.nextInt();
		int num2 = input.nextInt();
		
		if((num1+num2)%2 == 0)
			System.out.println((num1+num2)/2);
		else
			System.out.println("IMPOSSIBLE");
		
		input.close();

	}

} Main public static void main(String[] args) {
		Scanner input = new Scanner(System.in);
		int num1 = input.nextInt();
		int num2 = input.nextInt();
		
		if((num1+num2)%2 == 0)
			System.out.println((num1+num2)/2);
		else
			System.out.println("IMPOSSIBLE");
		
		input.close();

	} main {
		Scanner input = new Scanner(System.in);
		int num1 = input.nextInt();
		int num2 = input.nextInt();
		
		if((num1+num2)%2 == 0)
			System.out.println((num1+num2)/2);
		else
			System.out.println("IMPOSSIBLE");
		
		input.close();

	} Scanner input = new Scanner(System.in); input new Scanner(System.in) int num1 = input.nextInt(); num1 input.nextInt() input.nextInt input int num2 = input.nextInt(); num2 input.nextInt() input.nextInt input if((num1+num2)%2 == 0)
			System.out.println((num1+num2)/2);
		else
			System.out.println("IMPOSSIBLE"); (num1+num2)%2 == 0 (num1+num2)%2 (num1+num2) num1 num2 2 0 System.out.println((num1+num2)/2) System.out.println System.out System System.out (num1+num2)/2 (num1+num2) num1 num2 2 System.out.println("IMPOSSIBLE") System.out.println System.out System System.out "IMPOSSIBLE" input.close() input.close input String[] args args public class Main {

	public static void main(String[] args) {
		Scanner input = new Scanner(System.in);
		int num1 = input.nextInt();
		int num2 = input.nextInt();
		
		if((num1+num2)%2 == 0)
			System.out.println((num1+num2)/2);
		else
			System.out.println("IMPOSSIBLE");
		
		input.close();

	}

} public class Main {

	public static void main(String[] args) {
		Scanner input = new Scanner(System.in);
		int num1 = input.nextInt();
		int num2 = input.nextInt();
		
		if((num1+num2)%2 == 0)
			System.out.println((num1+num2)/2);
		else
			System.out.println("IMPOSSIBLE");
		
		input.close();

	}

} Main
import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		Scanner input = new Scanner(System.in);
		int num1 = input.nextInt();
		int num2 = input.nextInt();
		
		if((num1+num2)%2 == 0)
			System.out.println((num1+num2)/2);
		else
			System.out.println("IMPOSSIBLE");
		
		input.close();

	}

}
