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=0;
    int s=0;
    int s1;
    int s2;
    int k1;
    int k2;
    int ans=0;

    if(a>b){
      k=a;
      s=b;
    }else{
      k=b;
      s=a;
    }
    for(int i=s;i<k;i++){
      s1=a-i;
      s2=i-a;
      k1=b-i;
      k2=i-b;
      if(s1==k1){
        System.out.println(i);
        ans=1;
      }else if(s1==k2){
        System.out.println(i);
        ans=1;
      }else if(s2==k1){
        System.out.println(i);
        ans=1;
      }else if(s2==k2){
        System.out.println(i);
        ans=1;
      }
    }
    if(ans!=1){
      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=0;
    int s=0;
    int s1;
    int s2;
    int k1;
    int k2;
    int ans=0;

    if(a>b){
      k=a;
      s=b;
    }else{
      k=b;
      s=a;
    }
    for(int i=s;i<k;i++){
      s1=a-i;
      s2=i-a;
      k1=b-i;
      k2=i-b;
      if(s1==k1){
        System.out.println(i);
        ans=1;
      }else if(s1==k2){
        System.out.println(i);
        ans=1;
      }else if(s2==k1){
        System.out.println(i);
        ans=1;
      }else if(s2==k2){
        System.out.println(i);
        ans=1;
      }
    }
    if(ans!=1){
      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=0;
    int s=0;
    int s1;
    int s2;
    int k1;
    int k2;
    int ans=0;

    if(a>b){
      k=a;
      s=b;
    }else{
      k=b;
      s=a;
    }
    for(int i=s;i<k;i++){
      s1=a-i;
      s2=i-a;
      k1=b-i;
      k2=i-b;
      if(s1==k1){
        System.out.println(i);
        ans=1;
      }else if(s1==k2){
        System.out.println(i);
        ans=1;
      }else if(s2==k1){
        System.out.println(i);
        ans=1;
      }else if(s2==k2){
        System.out.println(i);
        ans=1;
      }
    }
    if(ans!=1){
      System.out.println("IMPOSSIBLE");
    }

  } main {
    Scanner sc=new Scanner(System.in);
    int a=sc.nextInt();
    int b=sc.nextInt();
    int k=0;
    int s=0;
    int s1;
    int s2;
    int k1;
    int k2;
    int ans=0;

    if(a>b){
      k=a;
      s=b;
    }else{
      k=b;
      s=a;
    }
    for(int i=s;i<k;i++){
      s1=a-i;
      s2=i-a;
      k1=b-i;
      k2=i-b;
      if(s1==k1){
        System.out.println(i);
        ans=1;
      }else if(s1==k2){
        System.out.println(i);
        ans=1;
      }else if(s2==k1){
        System.out.println(i);
        ans=1;
      }else if(s2==k2){
        System.out.println(i);
        ans=1;
      }
    }
    if(ans!=1){
      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=0; k 0 int s=0; s 0 int s1; s1 int s2; s2 int k1; k1 int k2; k2 int ans=0; ans 0 if(a>b){
      k=a;
      s=b;
    }else{
      k=b;
      s=a;
    } a>b a b {
      k=a;
      s=b;
    } k=a k a s=b s b {
      k=b;
      s=a;
    } k=b k b s=a s a for(int i=s;i<k;i++){
      s1=a-i;
      s2=i-a;
      k1=b-i;
      k2=i-b;
      if(s1==k1){
        System.out.println(i);
        ans=1;
      }else if(s1==k2){
        System.out.println(i);
        ans=1;
      }else if(s2==k1){
        System.out.println(i);
        ans=1;
      }else if(s2==k2){
        System.out.println(i);
        ans=1;
      }
    } int i=s; int i=s; i s i<k i k i++ i++ i {
      s1=a-i;
      s2=i-a;
      k1=b-i;
      k2=i-b;
      if(s1==k1){
        System.out.println(i);
        ans=1;
      }else if(s1==k2){
        System.out.println(i);
        ans=1;
      }else if(s2==k1){
        System.out.println(i);
        ans=1;
      }else if(s2==k2){
        System.out.println(i);
        ans=1;
      }
    } {
      s1=a-i;
      s2=i-a;
      k1=b-i;
      k2=i-b;
      if(s1==k1){
        System.out.println(i);
        ans=1;
      }else if(s1==k2){
        System.out.println(i);
        ans=1;
      }else if(s2==k1){
        System.out.println(i);
        ans=1;
      }else if(s2==k2){
        System.out.println(i);
        ans=1;
      }
    } s1=a-i s1 a-i a i s2=i-a s2 i-a i a k1=b-i k1 b-i b i k2=i-b k2 i-b i b if(s1==k1){
        System.out.println(i);
        ans=1;
      }else if(s1==k2){
        System.out.println(i);
        ans=1;
      }else if(s2==k1){
        System.out.println(i);
        ans=1;
      }else if(s2==k2){
        System.out.println(i);
        ans=1;
      } s1==k1 s1 k1 {
        System.out.println(i);
        ans=1;
      } System.out.println(i) System.out.println System.out System System.out i ans=1 ans 1 if(s1==k2){
        System.out.println(i);
        ans=1;
      }else if(s2==k1){
        System.out.println(i);
        ans=1;
      }else if(s2==k2){
        System.out.println(i);
        ans=1;
      } s1==k2 s1 k2 {
        System.out.println(i);
        ans=1;
      } System.out.println(i) System.out.println System.out System System.out i ans=1 ans 1 if(s2==k1){
        System.out.println(i);
        ans=1;
      }else if(s2==k2){
        System.out.println(i);
        ans=1;
      } s2==k1 s2 k1 {
        System.out.println(i);
        ans=1;
      } System.out.println(i) System.out.println System.out System System.out i ans=1 ans 1 if(s2==k2){
        System.out.println(i);
        ans=1;
      } s2==k2 s2 k2 {
        System.out.println(i);
        ans=1;
      } System.out.println(i) System.out.println System.out System System.out i ans=1 ans 1 if(ans!=1){
      System.out.println("IMPOSSIBLE");
    } ans!=1 ans 1 {
      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=0;
    int s=0;
    int s1;
    int s2;
    int k1;
    int k2;
    int ans=0;

    if(a>b){
      k=a;
      s=b;
    }else{
      k=b;
      s=a;
    }
    for(int i=s;i<k;i++){
      s1=a-i;
      s2=i-a;
      k1=b-i;
      k2=i-b;
      if(s1==k1){
        System.out.println(i);
        ans=1;
      }else if(s1==k2){
        System.out.println(i);
        ans=1;
      }else if(s2==k1){
        System.out.println(i);
        ans=1;
      }else if(s2==k2){
        System.out.println(i);
        ans=1;
      }
    }
    if(ans!=1){
      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=0;
    int s=0;
    int s1;
    int s2;
    int k1;
    int k2;
    int ans=0;

    if(a>b){
      k=a;
      s=b;
    }else{
      k=b;
      s=a;
    }
    for(int i=s;i<k;i++){
      s1=a-i;
      s2=i-a;
      k1=b-i;
      k2=i-b;
      if(s1==k1){
        System.out.println(i);
        ans=1;
      }else if(s1==k2){
        System.out.println(i);
        ans=1;
      }else if(s2==k1){
        System.out.println(i);
        ans=1;
      }else if(s2==k2){
        System.out.println(i);
        ans=1;
      }
    }
    if(ans!=1){
      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=0;
    int s=0;
    int s1;
    int s2;
    int k1;
    int k2;
    int ans=0;

    if(a>b){
      k=a;
      s=b;
    }else{
      k=b;
      s=a;
    }
    for(int i=s;i<k;i++){
      s1=a-i;
      s2=i-a;
      k1=b-i;
      k2=i-b;
      if(s1==k1){
        System.out.println(i);
        ans=1;
      }else if(s1==k2){
        System.out.println(i);
        ans=1;
      }else if(s2==k1){
        System.out.println(i);
        ans=1;
      }else if(s2==k2){
        System.out.println(i);
        ans=1;
      }
    }
    if(ans!=1){
      System.out.println("IMPOSSIBLE");
    }

  }
}
