import java.util.Scanner;

public class Main { 
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		String[] line = sc.nextLine().split(" ");
		long num1 = Long.parseLong(line[0]);
		long num2 = Long.parseLong(line[1]);

		if(num1%2 == num2%2) {
			long num3 = num1 + num2 ;
			System.out.println(num3/2);
		}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