import java.util.Scanner;
class Main{
    public static void main(String[] args) {
        Scanner scn=new Scanner(System.in);
        int a=scn.nextInt();
        int b=scn.nextInt();
        int answer=0;
        int max =Math.max(a,b);
        int min = Math.min(a,b);
        for(int i=0;;i++){
            if(max<=0&&min<=0||(a+b)%2==1){
                System.out.println("IMPOSSIBLE");
                break;
            }
            if(Math.abs(max)==Math.abs(min)){
                System.out.println(answer);
                break;
            }
            max--;
            min--;
            answer++;
        }
    }
}

/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