import java.util.Scanner;

public class Main {
  public static void main(String[] args) {
    Scanner scan = new Scanner(System.in);
    
    String strA = scan.next();
    String strB = scan.next();
    
    int a = Integer.parseInt(strA);
    int b = Integer.parseInt(strB);
    
    int c = a + b;
    
    if (c % 2 == 0) {
      System.out.print(c / 2);
    } else {
      System.out.print("IMPOSSIBLE");
    }
  }
} import java.util.Scanner; Scanner java.util public class Main {
  public static void main(String[] args) {
    Scanner scan = new Scanner(System.in);
    
    String strA = scan.next();
    String strB = scan.next();
    
    int a = Integer.parseInt(strA);
    int b = Integer.parseInt(strB);
    
    int c = a + b;
    
    if (c % 2 == 0) {
      System.out.print(c / 2);
    } else {
      System.out.print("IMPOSSIBLE");
    }
  }
} Main public static void main(String[] args) {
    Scanner scan = new Scanner(System.in);
    
    String strA = scan.next();
    String strB = scan.next();
    
    int a = Integer.parseInt(strA);
    int b = Integer.parseInt(strB);
    
    int c = a + b;
    
    if (c % 2 == 0) {
      System.out.print(c / 2);
    } else {
      System.out.print("IMPOSSIBLE");
    }
  } main {
    Scanner scan = new Scanner(System.in);
    
    String strA = scan.next();
    String strB = scan.next();
    
    int a = Integer.parseInt(strA);
    int b = Integer.parseInt(strB);
    
    int c = a + b;
    
    if (c % 2 == 0) {
      System.out.print(c / 2);
    } else {
      System.out.print("IMPOSSIBLE");
    }
  } Scanner scan = new Scanner(System.in); scan new Scanner(System.in) String strA = scan.next(); strA scan.next() scan.next scan String strB = scan.next(); strB scan.next() scan.next scan int a = Integer.parseInt(strA); a Integer.parseInt(strA) Integer.parseInt Integer strA int b = Integer.parseInt(strB); b Integer.parseInt(strB) Integer.parseInt Integer strB int c = a + b; c a + b a b if (c % 2 == 0) {
      System.out.print(c / 2);
    } else {
      System.out.print("IMPOSSIBLE");
    } c % 2 == 0 c % 2 c 2 0 {
      System.out.print(c / 2);
    } System.out.print(c / 2) System.out.print System.out System System.out c / 2 c 2 {
      System.out.print("IMPOSSIBLE");
    } System.out.print("IMPOSSIBLE") System.out.print System.out System System.out "IMPOSSIBLE" String[] args args public class Main {
  public static void main(String[] args) {
    Scanner scan = new Scanner(System.in);
    
    String strA = scan.next();
    String strB = scan.next();
    
    int a = Integer.parseInt(strA);
    int b = Integer.parseInt(strB);
    
    int c = a + b;
    
    if (c % 2 == 0) {
      System.out.print(c / 2);
    } else {
      System.out.print("IMPOSSIBLE");
    }
  }
} public class Main {
  public static void main(String[] args) {
    Scanner scan = new Scanner(System.in);
    
    String strA = scan.next();
    String strB = scan.next();
    
    int a = Integer.parseInt(strA);
    int b = Integer.parseInt(strB);
    
    int c = a + b;
    
    if (c % 2 == 0) {
      System.out.print(c / 2);
    } else {
      System.out.print("IMPOSSIBLE");
    }
  }
} Main
import java.util.Scanner;

public class Main {
  public static void main(String[] args) {
    Scanner scan = new Scanner(System.in);
    
    String strA = scan.next();
    String strB = scan.next();
    
    int a = Integer.parseInt(strA);
    int b = Integer.parseInt(strB);
    
    int c = a + b;
    
    if (c % 2 == 0) {
      System.out.print(c / 2);
    } else {
      System.out.print("IMPOSSIBLE");
    }
  }
}