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 x = (a+b)/2;
    if(a+b == x*2){
      System.out.println(x);
    }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 x = (a+b)/2;
    if(a+b == x*2){
      System.out.println(x);
    }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 x = (a+b)/2;
    if(a+b == x*2){
      System.out.println(x);
    }else{
      System.out.println("IMPOSSIBLE");
    }
  } main {
    Scanner sc = new Scanner(System.in);
    int a = sc.nextInt();
    int b = sc.nextInt();
    int x = (a+b)/2;
    if(a+b == x*2){
      System.out.println(x);
    }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 x = (a+b)/2; x (a+b)/2 (a+b) a b 2 if(a+b == x*2){
      System.out.println(x);
    }else{
      System.out.println("IMPOSSIBLE");
    } a+b == x*2 a+b a b x*2 x 2 {
      System.out.println(x);
    } System.out.println(x) System.out.println System.out System System.out x {
      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 x = (a+b)/2;
    if(a+b == x*2){
      System.out.println(x);
    }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 x = (a+b)/2;
    if(a+b == x*2){
      System.out.println(x);
    }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 x = (a+b)/2;
    if(a+b == x*2){
      System.out.println(x);
    }else{
      System.out.println("IMPOSSIBLE");
    }
  }
}
