import java.util.Scanner;
public class Main {
	public static void main(String[] args)throws Exception{
		Scanner stdIn=new Scanner(System.in);
		int A=stdIn.nextInt();
		int B=stdIn.nextInt();
		if((A+B)%2==0)
			System.out.println((A+B)/2);
		else
			System.out.println("IMPOSSIBLE");
	}
} import java.util.Scanner; Scanner java.util public class Main {
	public static void main(String[] args)throws Exception{
		Scanner stdIn=new Scanner(System.in);
		int A=stdIn.nextInt();
		int B=stdIn.nextInt();
		if((A+B)%2==0)
			System.out.println((A+B)/2);
		else
			System.out.println("IMPOSSIBLE");
	}
} Main public static void main(String[] args)throws Exception{
		Scanner stdIn=new Scanner(System.in);
		int A=stdIn.nextInt();
		int B=stdIn.nextInt();
		if((A+B)%2==0)
			System.out.println((A+B)/2);
		else
			System.out.println("IMPOSSIBLE");
	} main {
		Scanner stdIn=new Scanner(System.in);
		int A=stdIn.nextInt();
		int B=stdIn.nextInt();
		if((A+B)%2==0)
			System.out.println((A+B)/2);
		else
			System.out.println("IMPOSSIBLE");
	} Scanner stdIn=new Scanner(System.in); stdIn new Scanner(System.in) int A=stdIn.nextInt(); A stdIn.nextInt() stdIn.nextInt stdIn int B=stdIn.nextInt(); B stdIn.nextInt() stdIn.nextInt stdIn if((A+B)%2==0)
			System.out.println((A+B)/2);
		else
			System.out.println("IMPOSSIBLE"); (A+B)%2==0 (A+B)%2 (A+B) A B 2 0 System.out.println((A+B)/2) System.out.println System.out System System.out (A+B)/2 (A+B) A B 2 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)throws Exception{
		Scanner stdIn=new Scanner(System.in);
		int A=stdIn.nextInt();
		int B=stdIn.nextInt();
		if((A+B)%2==0)
			System.out.println((A+B)/2);
		else
			System.out.println("IMPOSSIBLE");
	}
} public class Main {
	public static void main(String[] args)throws Exception{
		Scanner stdIn=new Scanner(System.in);
		int A=stdIn.nextInt();
		int B=stdIn.nextInt();
		if((A+B)%2==0)
			System.out.println((A+B)/2);
		else
			System.out.println("IMPOSSIBLE");
	}
} Main
import java.util.Scanner;
public class Main {
	public static void main(String[] args)throws Exception{
		Scanner stdIn=new Scanner(System.in);
		int A=stdIn.nextInt();
		int B=stdIn.nextInt();
		if((A+B)%2==0)
			System.out.println((A+B)/2);
		else
			System.out.println("IMPOSSIBLE");
	}
}
