import java.util.*;
public class Main{
  public static void main(String args[]){
    Scanner sc=new Scanner(System.in);
    int n=sc.nextInt();
    int m=sc.nextInt();
    int x=Math.abs(m-n);
    int y=0;

    if(x%2==0){
       y=x/2;
         if (n<m) {
         System.out.println(n+y);
       }else{
         System.out.println(m+y);
       }
     }

    if(x%2!=0){
      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