import java.io.*;
import java.util.*;

public class Main {
 	public static void main(String[] args) throws Exception {
     	Scanner sc = new Scanner(System.in);
      	PrintWriter out = new PrintWriter(System.out);
      	int a = sc.nextInt();
      	int b = sc.nextInt();
      	if ((a + b) % 2 == 0) {
         	out.println((a+b) / 2); 
        }
      	else {
         	out.println("IMPOSSIBLE"); 
        }
      	out.close();
    }
}
/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