import java.util.Scanner;

public class Main{
        public  static void main(String[] args){
                calc();
        }
        static int calc(){
                Scanner sc = new Scanner(System.in);
                int a=sc.nextInt();
                int b=sc.nextInt();
                int c,d;
                if((a+b)%2==0){
                        c=(a+b)/2;
                        if(c>0){
                                System.out.println(c);
                        return c;
                        }else{
                                d =c*(-1);
                                System.out.println(d);
                                return d;
                        }
                }else{
                        System.out.println("IMPOSSIBLE");
                }
                return 0;
        }
}
/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