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");
	   }
   }
}

/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