import java.util.*;
public class Main{
  public static void main(String args[]){
  Scanner sc = new Scanner(System.in);
  int a = sc.nextInt();
  int b = sc.nextInt();
  int x = a%2;
  int y = b%2;
  if(a>b){
      if((x==0 && y==0)||(x!=0 && y!=0)){
        int ak = (a-b)/2;
        int k = a-ak;
        System.out.println(k);
      }else{
        System.out.println("IMPOSSIBLE");
      }
  }else{
      if((x==0 && y==0)||(x!=0 && y!=0)){
        int bk = (b-a)/2;
        int k = b-bk;
        System.out.println(k);
      }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