import java.io.*;

public class Main {

  public static void main(String[] args){

    try{
      String str;
      BufferedReader br = new BufferedReader(new InputStreamReader(System.in, "UTF-8"));
      str = br.readLine();
String[] ss = str.split(" ");
      //int sample = Integer.parseInt(str);
      func(Integer.parseInt(ss[0]),Integer.parseInt(ss[1]));
    } catch(IOException e){
      e.printStackTrace();
    }
  }

  public static void func(int a, int b){
    if((a+b)%2 == 0){
System.out.println(Math.abs((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