import java.util.Scanner;
public class Main {
	
	
	public static int findK(int x)
	{
		int k= x/2;
		System.out.println(k);
		return k;
	}
	
	public static void main(String[] args)
	{
		Scanner scann = new Scanner(System.in); 
		int a=scann.nextInt();
		int b=scann.nextInt();
		
		
		int x= a+b;
		if(a==b)
			System.out.println("IMPOSSIBLE");
		else if(x%2==1)
			System.out.println("IMPOSSIBLE");
		else
			 findK(x);
			;
			
			

}
} import java.util.Scanner; Scanner java.util public class Main {
	
	
	public static int findK(int x)
	{
		int k= x/2;
		System.out.println(k);
		return k;
	}
	
	public static void main(String[] args)
	{
		Scanner scann = new Scanner(System.in); 
		int a=scann.nextInt();
		int b=scann.nextInt();
		
		
		int x= a+b;
		if(a==b)
			System.out.println("IMPOSSIBLE");
		else if(x%2==1)
			System.out.println("IMPOSSIBLE");
		else
			 findK(x);
			;
			
			

}
} Main public static int findK(int x)
	{
		int k= x/2;
		System.out.println(k);
		return k;
	} findK {
		int k= x/2;
		System.out.println(k);
		return k;
	} int k= x/2; k x/2 x 2 System.out.println(k) System.out.println System.out System System.out k return k; k int x x public static void main(String[] args)
	{
		Scanner scann = new Scanner(System.in); 
		int a=scann.nextInt();
		int b=scann.nextInt();
		
		
		int x= a+b;
		if(a==b)
			System.out.println("IMPOSSIBLE");
		else if(x%2==1)
			System.out.println("IMPOSSIBLE");
		else
			 findK(x);
			;
			
			

} main {
		Scanner scann = new Scanner(System.in); 
		int a=scann.nextInt();
		int b=scann.nextInt();
		
		
		int x= a+b;
		if(a==b)
			System.out.println("IMPOSSIBLE");
		else if(x%2==1)
			System.out.println("IMPOSSIBLE");
		else
			 findK(x);
			;
			
			

} Scanner scann = new Scanner(System.in); scann new Scanner(System.in) int a=scann.nextInt(); a scann.nextInt() scann.nextInt scann int b=scann.nextInt(); b scann.nextInt() scann.nextInt scann int x= a+b; x a+b a b if(a==b)
			System.out.println("IMPOSSIBLE");
		else if(x%2==1)
			System.out.println("IMPOSSIBLE");
		else
			 findK(x); a==b a b System.out.println("IMPOSSIBLE") System.out.println System.out System System.out "IMPOSSIBLE" if(x%2==1)
			System.out.println("IMPOSSIBLE");
		else
			 findK(x); x%2==1 x%2 x 2 1 System.out.println("IMPOSSIBLE") System.out.println System.out System System.out "IMPOSSIBLE" findK(x) findK x String[] args args public class Main {
	
	
	public static int findK(int x)
	{
		int k= x/2;
		System.out.println(k);
		return k;
	}
	
	public static void main(String[] args)
	{
		Scanner scann = new Scanner(System.in); 
		int a=scann.nextInt();
		int b=scann.nextInt();
		
		
		int x= a+b;
		if(a==b)
			System.out.println("IMPOSSIBLE");
		else if(x%2==1)
			System.out.println("IMPOSSIBLE");
		else
			 findK(x);
			;
			
			

}
} public class Main {
	
	
	public static int findK(int x)
	{
		int k= x/2;
		System.out.println(k);
		return k;
	}
	
	public static void main(String[] args)
	{
		Scanner scann = new Scanner(System.in); 
		int a=scann.nextInt();
		int b=scann.nextInt();
		
		
		int x= a+b;
		if(a==b)
			System.out.println("IMPOSSIBLE");
		else if(x%2==1)
			System.out.println("IMPOSSIBLE");
		else
			 findK(x);
			;
			
			

}
} Main
import java.util.Scanner;
public class Main {
	
	
	public static int findK(int x)
	{
		int k= x/2;
		System.out.println(k);
		return k;
	}
	
	public static void main(String[] args)
	{
		Scanner scann = new Scanner(System.in); 
		int a=scann.nextInt();
		int b=scann.nextInt();
		
		
		int x= a+b;
		if(a==b)
			System.out.println("IMPOSSIBLE");
		else if(x%2==1)
			System.out.println("IMPOSSIBLE");
		else
			 findK(x);
			;
			
			

}
}
