import java.util.Scanner;

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

public class Main {
   public static void main(String[] args) {
	   Scanner Ahmad=new Scanner(System.in);
	   int x,y;
	   x=Ahmad.nextInt();
	   y=Ahmad.nextInt();
	   if((x+y)%2==0) {
		   System.out.println((x+y)/2);
	   }
	   else {
		   System.out.println("IMPOSSIBLE");
	   }
   }
}
