import java.util.*;
import java.util.InputMismatchException;
public class Main {
	public static void main(String[] args){
      try{
		Scanner sc = new Scanner(System.in);
     
	      int a = sc.nextInt();
	      int b = sc.nextInt();
	      int k;
      
	      if(a == b){
 	  	     System.out.println("IMPOSSIBLE");
          }else if(a < 0){
            System.out.println("IMPOSSIBLE");
          }else if (b > 1000000000){
            System.out.println("IMPOSSIBLE");
	      }else{
	        k=(b+a)/2;
            if((k*2) != (a+b)){
              System.out.println("IMPOSSIBLE");
            }else{
			System.out.println(k);
            }
	      } 
      }catch(InputMismatchException e){
        System.out.println("IMPOSSIBLE");
      }
	}
}

/tmp/hf-datasets-cache/medium/datasets/42704881117311-config-parquet-and-info-qiankunmu-Project_CodeNet-896c9d36/hub/datasets--qiankunmu--Project_CodeNet_Python800_and_Java250/snapshots/c899fde44a77ba1cc14973ed3de064e4ff3aadc7/Project_CodeNet_Java250.tar.gz