import java.util.*;
 
public class Main {
  public static void main (String[] arg) {
    Scanner scan = new Scanner(System.in);
    int A = scan.nextInt();
    int B = scan.nextInt();
    int K = 0;
    int check = 0;
    
    if((A+B)%2 == 0){
      K = (Math.abs(A)+Math.abs(B))/2;
      check++;
    }
    
    if(check == 0){
      System.out.println("IMPOSSIBLE");
    } else {
      System.out.println(K);
    }
    
  }
} import java.util.*; util.* java public class Main {
  public static void main (String[] arg) {
    Scanner scan = new Scanner(System.in);
    int A = scan.nextInt();
    int B = scan.nextInt();
    int K = 0;
    int check = 0;
    
    if((A+B)%2 == 0){
      K = (Math.abs(A)+Math.abs(B))/2;
      check++;
    }
    
    if(check == 0){
      System.out.println("IMPOSSIBLE");
    } else {
      System.out.println(K);
    }
    
  }
} Main public static void main (String[] arg) {
    Scanner scan = new Scanner(System.in);
    int A = scan.nextInt();
    int B = scan.nextInt();
    int K = 0;
    int check = 0;
    
    if((A+B)%2 == 0){
      K = (Math.abs(A)+Math.abs(B))/2;
      check++;
    }
    
    if(check == 0){
      System.out.println("IMPOSSIBLE");
    } else {
      System.out.println(K);
    }
    
  } main {
    Scanner scan = new Scanner(System.in);
    int A = scan.nextInt();
    int B = scan.nextInt();
    int K = 0;
    int check = 0;
    
    if((A+B)%2 == 0){
      K = (Math.abs(A)+Math.abs(B))/2;
      check++;
    }
    
    if(check == 0){
      System.out.println("IMPOSSIBLE");
    } else {
      System.out.println(K);
    }
    
  } Scanner scan = new Scanner(System.in); scan new Scanner(System.in) int A = scan.nextInt(); A scan.nextInt() scan.nextInt scan int B = scan.nextInt(); B scan.nextInt() scan.nextInt scan int K = 0; K 0 int check = 0; check 0 if((A+B)%2 == 0){
      K = (Math.abs(A)+Math.abs(B))/2;
      check++;
    } (A+B)%2 == 0 (A+B)%2 (A+B) A B 2 0 {
      K = (Math.abs(A)+Math.abs(B))/2;
      check++;
    } K = (Math.abs(A)+Math.abs(B))/2 K (Math.abs(A)+Math.abs(B))/2 (Math.abs(A)+Math.abs(B)) Math.abs(A) Math.abs Math A Math.abs(B) Math.abs Math B 2 check++ check if(check == 0){
      System.out.println("IMPOSSIBLE");
    } else {
      System.out.println(K);
    } check == 0 check 0 {
      System.out.println("IMPOSSIBLE");
    } System.out.println("IMPOSSIBLE") System.out.println System.out System System.out "IMPOSSIBLE" {
      System.out.println(K);
    } System.out.println(K) System.out.println System.out System System.out K String[] arg arg public class Main {
  public static void main (String[] arg) {
    Scanner scan = new Scanner(System.in);
    int A = scan.nextInt();
    int B = scan.nextInt();
    int K = 0;
    int check = 0;
    
    if((A+B)%2 == 0){
      K = (Math.abs(A)+Math.abs(B))/2;
      check++;
    }
    
    if(check == 0){
      System.out.println("IMPOSSIBLE");
    } else {
      System.out.println(K);
    }
    
  }
} public class Main {
  public static void main (String[] arg) {
    Scanner scan = new Scanner(System.in);
    int A = scan.nextInt();
    int B = scan.nextInt();
    int K = 0;
    int check = 0;
    
    if((A+B)%2 == 0){
      K = (Math.abs(A)+Math.abs(B))/2;
      check++;
    }
    
    if(check == 0){
      System.out.println("IMPOSSIBLE");
    } else {
      System.out.println(K);
    }
    
  }
} Main
import java.util.*;
 
public class Main {
  public static void main (String[] arg) {
    Scanner scan = new Scanner(System.in);
    int A = scan.nextInt();
    int B = scan.nextInt();
    int K = 0;
    int check = 0;
    
    if((A+B)%2 == 0){
      K = (Math.abs(A)+Math.abs(B))/2;
      check++;
    }
    
    if(check == 0){
      System.out.println("IMPOSSIBLE");
    } else {
      System.out.println(K);
    }
    
  }
}