import java.util.*;


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


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