import java.util.Scanner;

public class Main{
  public static void main(String[] args){
    Scanner sc=new Scanner(System.in);
    int a=sc.nextInt();
    int b=sc.nextInt();
    boolean flg=false;
    long k;
    for(k=0;k<1000000000;k++){
      if(Math.abs(a-k)==Math.abs(b-k)){
        flg=true;
        break;
      }
    }
    if(flg){
      System.out.println(k);
    }else{
      System.out.println("IMPOSSIBLE");
    }
  }
} import java.util.Scanner; Scanner 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();
    boolean flg=false;
    long k;
    for(k=0;k<1000000000;k++){
      if(Math.abs(a-k)==Math.abs(b-k)){
        flg=true;
        break;
      }
    }
    if(flg){
      System.out.println(k);
    }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();
    boolean flg=false;
    long k;
    for(k=0;k<1000000000;k++){
      if(Math.abs(a-k)==Math.abs(b-k)){
        flg=true;
        break;
      }
    }
    if(flg){
      System.out.println(k);
    }else{
      System.out.println("IMPOSSIBLE");
    }
  } main {
    Scanner sc=new Scanner(System.in);
    int a=sc.nextInt();
    int b=sc.nextInt();
    boolean flg=false;
    long k;
    for(k=0;k<1000000000;k++){
      if(Math.abs(a-k)==Math.abs(b-k)){
        flg=true;
        break;
      }
    }
    if(flg){
      System.out.println(k);
    }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 boolean flg=false; flg false long k; k for(k=0;k<1000000000;k++){
      if(Math.abs(a-k)==Math.abs(b-k)){
        flg=true;
        break;
      }
    } k=0; k=0 k 0 k<1000000000 k 1000000000 k++ k++ k {
      if(Math.abs(a-k)==Math.abs(b-k)){
        flg=true;
        break;
      }
    } {
      if(Math.abs(a-k)==Math.abs(b-k)){
        flg=true;
        break;
      }
    } if(Math.abs(a-k)==Math.abs(b-k)){
        flg=true;
        break;
      } Math.abs(a-k)==Math.abs(b-k) Math.abs(a-k) Math.abs Math a-k a k Math.abs(b-k) Math.abs Math b-k b k {
        flg=true;
        break;
      } flg=true flg true break; if(flg){
      System.out.println(k);
    }else{
      System.out.println("IMPOSSIBLE");
    } flg {
      System.out.println(k);
    } System.out.println(k) System.out.println System.out System System.out k {
      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();
    boolean flg=false;
    long k;
    for(k=0;k<1000000000;k++){
      if(Math.abs(a-k)==Math.abs(b-k)){
        flg=true;
        break;
      }
    }
    if(flg){
      System.out.println(k);
    }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();
    boolean flg=false;
    long k;
    for(k=0;k<1000000000;k++){
      if(Math.abs(a-k)==Math.abs(b-k)){
        flg=true;
        break;
      }
    }
    if(flg){
      System.out.println(k);
    }else{
      System.out.println("IMPOSSIBLE");
    }
  }
} Main
import java.util.Scanner;

public class Main{
  public static void main(String[] args){
    Scanner sc=new Scanner(System.in);
    int a=sc.nextInt();
    int b=sc.nextInt();
    boolean flg=false;
    long k;
    for(k=0;k<1000000000;k++){
      if(Math.abs(a-k)==Math.abs(b-k)){
        flg=true;
        break;
      }
    }
    if(flg){
      System.out.println(k);
    }else{
      System.out.println("IMPOSSIBLE");
    }
  }
}