import java.util.*;
class Main{
	public static void main(String[] args){
		int a , b;
		Scanner input = new Scanner(System.in);
		a = input.nextInt();
		b = input.nextInt();
		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