import java.util.* ;
class Main {
  public static void main(String[] args) {
    
    Scanner scan = new Scanner(System.in);

 long s = scan.nextLong();
    long d = scan.nextLong();
    //System.out.println(s+d);
    if(( s+d)  % 2 == 0){
      System.out.println((s+d)/2);
    }else{
      System.out.println("IMPOSSIBLE");
    }
    
  }
} import java.util.* ; util.* java class Main {
  public static void main(String[] args) {
    
    Scanner scan = new Scanner(System.in);

 long s = scan.nextLong();
    long d = scan.nextLong();
    //System.out.println(s+d);
    if(( s+d)  % 2 == 0){
      System.out.println((s+d)/2);
    }else{
      System.out.println("IMPOSSIBLE");
    }
    
  }
} Main public static void main(String[] args) {
    
    Scanner scan = new Scanner(System.in);

 long s = scan.nextLong();
    long d = scan.nextLong();
    //System.out.println(s+d);
    if(( s+d)  % 2 == 0){
      System.out.println((s+d)/2);
    }else{
      System.out.println("IMPOSSIBLE");
    }
    
  } main {
    
    Scanner scan = new Scanner(System.in);

 long s = scan.nextLong();
    long d = scan.nextLong();
    //System.out.println(s+d);
    if(( s+d)  % 2 == 0){
      System.out.println((s+d)/2);
    }else{
      System.out.println("IMPOSSIBLE");
    }
    
  } Scanner scan = new Scanner(System.in); scan new Scanner(System.in) long s = scan.nextLong(); s scan.nextLong() scan.nextLong scan long d = scan.nextLong(); d scan.nextLong() scan.nextLong scan if(( s+d)  % 2 == 0){
      System.out.println((s+d)/2);
    }else{
      System.out.println("IMPOSSIBLE");
    } ( s+d)  % 2 == 0 ( s+d)  % 2 ( s+d) s d 2 0 {
      System.out.println((s+d)/2);
    } System.out.println((s+d)/2) System.out.println System.out System System.out (s+d)/2 (s+d) s d 2 {
      System.out.println("IMPOSSIBLE");
    } System.out.println("IMPOSSIBLE") System.out.println System.out System System.out "IMPOSSIBLE" String[] args args
import java.util.* ;
class Main {
  public static void main(String[] args) {
    
    Scanner scan = new Scanner(System.in);

 long s = scan.nextLong();
    long d = scan.nextLong();
    //System.out.println(s+d);
    if(( s+d)  % 2 == 0){
      System.out.println((s+d)/2);
    }else{
      System.out.println("IMPOSSIBLE");
    }
    
  }
}