import java.util.Scanner;

public class Main {
    public static void main(String[]args){
        int a,b,sub1,sub2;
        Scanner in= new Scanner(System.in);
        a=in.nextInt(); b=in.nextInt();
        sub1=a+b;
        if(sub1%2==0)
            System.out.println(""+(sub1/2));
        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