import java.util.Scanner;

public class Main{

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Scanner input = new Scanner(System.in);
		
		int input1 = input.nextInt();
		int input2 = input.nextInt();
	 
		if ((input1 + input2) % 2 == 0)
			System.out.print((input1 + input2) / 2);
		else
			System.out.print("IMPOSSIBLE");
		
		
		input.close();

	}

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

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Scanner input = new Scanner(System.in);
		
		int input1 = input.nextInt();
		int input2 = input.nextInt();
	 
		if ((input1 + input2) % 2 == 0)
			System.out.print((input1 + input2) / 2);
		else
			System.out.print("IMPOSSIBLE");
		
		
		input.close();

	}

} Main public static void main(String[] args) {
		// TODO Auto-generated method stub
		Scanner input = new Scanner(System.in);
		
		int input1 = input.nextInt();
		int input2 = input.nextInt();
	 
		if ((input1 + input2) % 2 == 0)
			System.out.print((input1 + input2) / 2);
		else
			System.out.print("IMPOSSIBLE");
		
		
		input.close();

	} main {
		// TODO Auto-generated method stub
		Scanner input = new Scanner(System.in);
		
		int input1 = input.nextInt();
		int input2 = input.nextInt();
	 
		if ((input1 + input2) % 2 == 0)
			System.out.print((input1 + input2) / 2);
		else
			System.out.print("IMPOSSIBLE");
		
		
		input.close();

	} Scanner input = new Scanner(System.in); input new Scanner(System.in) int input1 = input.nextInt(); input1 input.nextInt() input.nextInt input int input2 = input.nextInt(); input2 input.nextInt() input.nextInt input if ((input1 + input2) % 2 == 0)
			System.out.print((input1 + input2) / 2);
		else
			System.out.print("IMPOSSIBLE"); (input1 + input2) % 2 == 0 (input1 + input2) % 2 (input1 + input2) input1 input2 2 0 System.out.print((input1 + input2) / 2) System.out.print System.out System System.out (input1 + input2) / 2 (input1 + input2) input1 input2 2 System.out.print("IMPOSSIBLE") System.out.print System.out System System.out "IMPOSSIBLE" input.close() input.close input String[] args args public class Main{

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Scanner input = new Scanner(System.in);
		
		int input1 = input.nextInt();
		int input2 = input.nextInt();
	 
		if ((input1 + input2) % 2 == 0)
			System.out.print((input1 + input2) / 2);
		else
			System.out.print("IMPOSSIBLE");
		
		
		input.close();

	}

} public class Main{

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Scanner input = new Scanner(System.in);
		
		int input1 = input.nextInt();
		int input2 = input.nextInt();
	 
		if ((input1 + input2) % 2 == 0)
			System.out.print((input1 + input2) / 2);
		else
			System.out.print("IMPOSSIBLE");
		
		
		input.close();

	}

} Main
import java.util.Scanner;

public class Main{

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Scanner input = new Scanner(System.in);
		
		int input1 = input.nextInt();
		int input2 = input.nextInt();
	 
		if ((input1 + input2) % 2 == 0)
			System.out.print((input1 + input2) / 2);
		else
			System.out.print("IMPOSSIBLE");
		
		
		input.close();

	}

}