import java.util.Scanner;
import java.util.InputMismatchException;

class Main {
   public static void main(String args[]) {
      int x=0 ,y=0;
      Scanner scanner = new Scanner(System.in);
      x = scanner.nextInt();
      y = scanner.nextInt();
      scanner.close();
      if ((x+y)%2 == 0) {
         int k1 = (x+y)/2;
         System.out.println(k1);
      }else if (x == y) {
         int k2 = x;
         System.out.println(k2);
      }else {
         System.out.println("IMPOSSIBLE");
      }

   }
} import java.util.Scanner; Scanner java.util import java.util.InputMismatchException; InputMismatchException java.util class Main {
   public static void main(String args[]) {
      int x=0 ,y=0;
      Scanner scanner = new Scanner(System.in);
      x = scanner.nextInt();
      y = scanner.nextInt();
      scanner.close();
      if ((x+y)%2 == 0) {
         int k1 = (x+y)/2;
         System.out.println(k1);
      }else if (x == y) {
         int k2 = x;
         System.out.println(k2);
      }else {
         System.out.println("IMPOSSIBLE");
      }

   }
} Main public static void main(String args[]) {
      int x=0 ,y=0;
      Scanner scanner = new Scanner(System.in);
      x = scanner.nextInt();
      y = scanner.nextInt();
      scanner.close();
      if ((x+y)%2 == 0) {
         int k1 = (x+y)/2;
         System.out.println(k1);
      }else if (x == y) {
         int k2 = x;
         System.out.println(k2);
      }else {
         System.out.println("IMPOSSIBLE");
      }

   } main {
      int x=0 ,y=0;
      Scanner scanner = new Scanner(System.in);
      x = scanner.nextInt();
      y = scanner.nextInt();
      scanner.close();
      if ((x+y)%2 == 0) {
         int k1 = (x+y)/2;
         System.out.println(k1);
      }else if (x == y) {
         int k2 = x;
         System.out.println(k2);
      }else {
         System.out.println("IMPOSSIBLE");
      }

   } int x=0 x 0 y=0; y 0 Scanner scanner = new Scanner(System.in); scanner new Scanner(System.in) x = scanner.nextInt() x scanner.nextInt() scanner.nextInt scanner y = scanner.nextInt() y scanner.nextInt() scanner.nextInt scanner scanner.close() scanner.close scanner if ((x+y)%2 == 0) {
         int k1 = (x+y)/2;
         System.out.println(k1);
      }else if (x == y) {
         int k2 = x;
         System.out.println(k2);
      }else {
         System.out.println("IMPOSSIBLE");
      } (x+y)%2 == 0 (x+y)%2 (x+y) x y 2 0 {
         int k1 = (x+y)/2;
         System.out.println(k1);
      } int k1 = (x+y)/2; k1 (x+y)/2 (x+y) x y 2 System.out.println(k1) System.out.println System.out System System.out k1 if (x == y) {
         int k2 = x;
         System.out.println(k2);
      }else {
         System.out.println("IMPOSSIBLE");
      } x == y x y {
         int k2 = x;
         System.out.println(k2);
      } int k2 = x; k2 x System.out.println(k2) System.out.println System.out System System.out k2 {
         System.out.println("IMPOSSIBLE");
      } System.out.println("IMPOSSIBLE") System.out.println System.out System System.out "IMPOSSIBLE" String args[] args
import java.util.Scanner;
import java.util.InputMismatchException;

class Main {
   public static void main(String args[]) {
      int x=0 ,y=0;
      Scanner scanner = new Scanner(System.in);
      x = scanner.nextInt();
      y = scanner.nextInt();
      scanner.close();
      if ((x+y)%2 == 0) {
         int k1 = (x+y)/2;
         System.out.println(k1);
      }else if (x == y) {
         int k2 = x;
         System.out.println(k2);
      }else {
         System.out.println("IMPOSSIBLE");
      }

   }
}
