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 k=a+b;

    if(k%2==0){
      if(k>=0){
        System.out.println(k/2);
      }else{
        System.out.println(-k/2);
      }
    }else{
      System.out.println("IMPOSSIBLE");
    }

  }
} import java.util.*; util.* java public class Main{
  public static void main(String[] args) {
    Scanner sc=new Scanner(System.in);
    int a=sc.nextInt();
    int b=sc.nextInt();
    int k=a+b;

    if(k%2==0){
      if(k>=0){
        System.out.println(k/2);
      }else{
        System.out.println(-k/2);
      }
    }else{
      System.out.println("IMPOSSIBLE");
    }

  }
} Main public static void main(String[] args) {
    Scanner sc=new Scanner(System.in);
    int a=sc.nextInt();
    int b=sc.nextInt();
    int k=a+b;

    if(k%2==0){
      if(k>=0){
        System.out.println(k/2);
      }else{
        System.out.println(-k/2);
      }
    }else{
      System.out.println("IMPOSSIBLE");
    }

  } main {
    Scanner sc=new Scanner(System.in);
    int a=sc.nextInt();
    int b=sc.nextInt();
    int k=a+b;

    if(k%2==0){
      if(k>=0){
        System.out.println(k/2);
      }else{
        System.out.println(-k/2);
      }
    }else{
      System.out.println("IMPOSSIBLE");
    }

  } Scanner sc=new Scanner(System.in); sc new Scanner(System.in) int a=sc.nextInt(); a sc.nextInt() sc.nextInt sc int b=sc.nextInt(); b sc.nextInt() sc.nextInt sc int k=a+b; k a+b a b if(k%2==0){
      if(k>=0){
        System.out.println(k/2);
      }else{
        System.out.println(-k/2);
      }
    }else{
      System.out.println("IMPOSSIBLE");
    } k%2==0 k%2 k 2 0 {
      if(k>=0){
        System.out.println(k/2);
      }else{
        System.out.println(-k/2);
      }
    } if(k>=0){
        System.out.println(k/2);
      }else{
        System.out.println(-k/2);
      } k>=0 k 0 {
        System.out.println(k/2);
      } System.out.println(k/2) System.out.println System.out System System.out k/2 k 2 {
        System.out.println(-k/2);
      } System.out.println(-k/2) System.out.println System.out System System.out -k/2 -k k 2 {
      System.out.println("IMPOSSIBLE");
    } System.out.println("IMPOSSIBLE") System.out.println System.out System System.out "IMPOSSIBLE" String[] args args public class Main{
  public static void main(String[] args) {
    Scanner sc=new Scanner(System.in);
    int a=sc.nextInt();
    int b=sc.nextInt();
    int k=a+b;

    if(k%2==0){
      if(k>=0){
        System.out.println(k/2);
      }else{
        System.out.println(-k/2);
      }
    }else{
      System.out.println("IMPOSSIBLE");
    }

  }
} public class Main{
  public static void main(String[] args) {
    Scanner sc=new Scanner(System.in);
    int a=sc.nextInt();
    int b=sc.nextInt();
    int k=a+b;

    if(k%2==0){
      if(k>=0){
        System.out.println(k/2);
      }else{
        System.out.println(-k/2);
      }
    }else{
      System.out.println("IMPOSSIBLE");
    }

  }
} Main
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 k=a+b;

    if(k%2==0){
      if(k>=0){
        System.out.println(k/2);
      }else{
        System.out.println(-k/2);
      }
    }else{
      System.out.println("IMPOSSIBLE");
    }

  }
}
