import java.util.*;

public class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int number_a = Integer.parseInt(sc.next());
    int number_b = Integer.parseInt(sc.next());
    if((number_a + number_b) % 2 == 1) {
      System.out.println("IMPOSSIBLE");
    } else {
      System.out.println((number_a + number_b) / 2);
    }
  }
} import java.util.*; util.* java public class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int number_a = Integer.parseInt(sc.next());
    int number_b = Integer.parseInt(sc.next());
    if((number_a + number_b) % 2 == 1) {
      System.out.println("IMPOSSIBLE");
    } else {
      System.out.println((number_a + number_b) / 2);
    }
  }
} Main public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int number_a = Integer.parseInt(sc.next());
    int number_b = Integer.parseInt(sc.next());
    if((number_a + number_b) % 2 == 1) {
      System.out.println("IMPOSSIBLE");
    } else {
      System.out.println((number_a + number_b) / 2);
    }
  } main {
    Scanner sc = new Scanner(System.in);
    int number_a = Integer.parseInt(sc.next());
    int number_b = Integer.parseInt(sc.next());
    if((number_a + number_b) % 2 == 1) {
      System.out.println("IMPOSSIBLE");
    } else {
      System.out.println((number_a + number_b) / 2);
    }
  } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int number_a = Integer.parseInt(sc.next()); number_a Integer.parseInt(sc.next()) Integer.parseInt Integer sc.next() sc.next sc int number_b = Integer.parseInt(sc.next()); number_b Integer.parseInt(sc.next()) Integer.parseInt Integer sc.next() sc.next sc if((number_a + number_b) % 2 == 1) {
      System.out.println("IMPOSSIBLE");
    } else {
      System.out.println((number_a + number_b) / 2);
    } (number_a + number_b) % 2 == 1 (number_a + number_b) % 2 (number_a + number_b) number_a number_b 2 1 {
      System.out.println("IMPOSSIBLE");
    } System.out.println("IMPOSSIBLE") System.out.println System.out System System.out "IMPOSSIBLE" {
      System.out.println((number_a + number_b) / 2);
    } System.out.println((number_a + number_b) / 2) System.out.println System.out System System.out (number_a + number_b) / 2 (number_a + number_b) number_a number_b 2 String[] args args public class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int number_a = Integer.parseInt(sc.next());
    int number_b = Integer.parseInt(sc.next());
    if((number_a + number_b) % 2 == 1) {
      System.out.println("IMPOSSIBLE");
    } else {
      System.out.println((number_a + number_b) / 2);
    }
  }
} public class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int number_a = Integer.parseInt(sc.next());
    int number_b = Integer.parseInt(sc.next());
    if((number_a + number_b) % 2 == 1) {
      System.out.println("IMPOSSIBLE");
    } else {
      System.out.println((number_a + number_b) / 2);
    }
  }
} Main
import java.util.*;

public class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int number_a = Integer.parseInt(sc.next());
    int number_b = Integer.parseInt(sc.next());
    if((number_a + number_b) % 2 == 1) {
      System.out.println("IMPOSSIBLE");
    } else {
      System.out.println((number_a + number_b) / 2);
    }
  }
}
