import java.util.*;

public class Main{

  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    long a = sc.nextLong();
    long b = sc.nextLong();

    long a1,a2=0,b1,b2=0;

    for(long i = 0;i < 1000000000;i++) {

      a1 = a-i;
      a2 = Math.abs(a1);
      b1 = b-i;
      b2 = Math.abs(b1);

      if(a2 == b2) {
        System.out.println(i);
        break;
      }

    }

    if(a2 != b2) {
      System.out.println("IMPOSSIBLE");
    }

  }

} import java.util.*; util.* java public class Main{

  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    long a = sc.nextLong();
    long b = sc.nextLong();

    long a1,a2=0,b1,b2=0;

    for(long i = 0;i < 1000000000;i++) {

      a1 = a-i;
      a2 = Math.abs(a1);
      b1 = b-i;
      b2 = Math.abs(b1);

      if(a2 == b2) {
        System.out.println(i);
        break;
      }

    }

    if(a2 != b2) {
      System.out.println("IMPOSSIBLE");
    }

  }

} Main public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    long a = sc.nextLong();
    long b = sc.nextLong();

    long a1,a2=0,b1,b2=0;

    for(long i = 0;i < 1000000000;i++) {

      a1 = a-i;
      a2 = Math.abs(a1);
      b1 = b-i;
      b2 = Math.abs(b1);

      if(a2 == b2) {
        System.out.println(i);
        break;
      }

    }

    if(a2 != b2) {
      System.out.println("IMPOSSIBLE");
    }

  } main {
    Scanner sc = new Scanner(System.in);
    long a = sc.nextLong();
    long b = sc.nextLong();

    long a1,a2=0,b1,b2=0;

    for(long i = 0;i < 1000000000;i++) {

      a1 = a-i;
      a2 = Math.abs(a1);
      b1 = b-i;
      b2 = Math.abs(b1);

      if(a2 == b2) {
        System.out.println(i);
        break;
      }

    }

    if(a2 != b2) {
      System.out.println("IMPOSSIBLE");
    }

  } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) long a = sc.nextLong(); a sc.nextLong() sc.nextLong sc long b = sc.nextLong(); b sc.nextLong() sc.nextLong sc long a1 a1 a2=0 a2 0 b1 b1 b2=0; b2 0 for(long i = 0;i < 1000000000;i++) {

      a1 = a-i;
      a2 = Math.abs(a1);
      b1 = b-i;
      b2 = Math.abs(b1);

      if(a2 == b2) {
        System.out.println(i);
        break;
      }

    } long i = 0; long i = 0; i 0 i < 1000000000 i 1000000000 i++ i++ i {

      a1 = a-i;
      a2 = Math.abs(a1);
      b1 = b-i;
      b2 = Math.abs(b1);

      if(a2 == b2) {
        System.out.println(i);
        break;
      }

    } {

      a1 = a-i;
      a2 = Math.abs(a1);
      b1 = b-i;
      b2 = Math.abs(b1);

      if(a2 == b2) {
        System.out.println(i);
        break;
      }

    } a1 = a-i a1 a-i a i a2 = Math.abs(a1) a2 Math.abs(a1) Math.abs Math a1 b1 = b-i b1 b-i b i b2 = Math.abs(b1) b2 Math.abs(b1) Math.abs Math b1 if(a2 == b2) {
        System.out.println(i);
        break;
      } a2 == b2 a2 b2 {
        System.out.println(i);
        break;
      } System.out.println(i) System.out.println System.out System System.out i break; if(a2 != b2) {
      System.out.println("IMPOSSIBLE");
    } a2 != b2 a2 b2 {
      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);
    long a = sc.nextLong();
    long b = sc.nextLong();

    long a1,a2=0,b1,b2=0;

    for(long i = 0;i < 1000000000;i++) {

      a1 = a-i;
      a2 = Math.abs(a1);
      b1 = b-i;
      b2 = Math.abs(b1);

      if(a2 == b2) {
        System.out.println(i);
        break;
      }

    }

    if(a2 != b2) {
      System.out.println("IMPOSSIBLE");
    }

  }

} public class Main{

  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    long a = sc.nextLong();
    long b = sc.nextLong();

    long a1,a2=0,b1,b2=0;

    for(long i = 0;i < 1000000000;i++) {

      a1 = a-i;
      a2 = Math.abs(a1);
      b1 = b-i;
      b2 = Math.abs(b1);

      if(a2 == b2) {
        System.out.println(i);
        break;
      }

    }

    if(a2 != b2) {
      System.out.println("IMPOSSIBLE");
    }

  }

} Main
import java.util.*;

public class Main{

  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    long a = sc.nextLong();
    long b = sc.nextLong();

    long a1,a2=0,b1,b2=0;

    for(long i = 0;i < 1000000000;i++) {

      a1 = a-i;
      a2 = Math.abs(a1);
      b1 = b-i;
      b2 = Math.abs(b1);

      if(a2 == b2) {
        System.out.println(i);
        break;
      }

    }

    if(a2 != b2) {
      System.out.println("IMPOSSIBLE");
    }

  }

}