import java.util.*; 

class Main{
public static void main(String[] args){
	Scanner sc=new Scanner(System.in);
        int a=sc.nextInt();
        int b=sc.nextInt();
        int ans=(a+b)/2;
    
 	if(a%2==1&&b%2==0){
    		System.out.println("IMPOSSIBLE");
    	}else if(a%2==0&&b%2==1){
     		System.out.println("IMPOSSIBLE");
   	 }else{

    	System.out.println(ans);
    }
    }
}

/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