import java.util.*;
public class Main {
	public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    String[] line = sc.nextLine().split(" ");
    int a = Integer.parseInt(line[0]);
    int b = Integer.parseInt(line[1]);
    if ((a + b) % 2 == 0) {
    	System.out.println((a + b) / 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