import java.util.*;

public class Main {
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        int b = sc.nextInt();
        
        if ((a % 2 == 0 && b % 2 == 0) || (a % 2 != 0 && b % 2 != 0)) {
          System.out.println(Math.abs(a+b)/2);
        } else {
          System.out.println("IMPOSSIBLE");
        }

    }
} import java.util.*; util.* java public class Main {
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        int b = sc.nextInt();
        
        if ((a % 2 == 0 && b % 2 == 0) || (a % 2 != 0 && b % 2 != 0)) {
          System.out.println(Math.abs(a+b)/2);
        } else {
          System.out.println("IMPOSSIBLE");
        }

    }
} Main public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        int b = sc.nextInt();
        
        if ((a % 2 == 0 && b % 2 == 0) || (a % 2 != 0 && b % 2 != 0)) {
          System.out.println(Math.abs(a+b)/2);
        } else {
          System.out.println("IMPOSSIBLE");
        }

    } main {
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        int b = sc.nextInt();
        
        if ((a % 2 == 0 && b % 2 == 0) || (a % 2 != 0 && b % 2 != 0)) {
          System.out.println(Math.abs(a+b)/2);
        } 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 if ((a % 2 == 0 && b % 2 == 0) || (a % 2 != 0 && b % 2 != 0)) {
          System.out.println(Math.abs(a+b)/2);
        } else {
          System.out.println("IMPOSSIBLE");
        } (a % 2 == 0 && b % 2 == 0) || (a % 2 != 0 && b % 2 != 0) (a % 2 == 0 && b % 2 == 0) a % 2 == 0 a % 2 a 2 0 b % 2 == 0 b % 2 b 2 0 (a % 2 != 0 && b % 2 != 0) a % 2 != 0 a % 2 a 2 0 b % 2 != 0 b % 2 b 2 0 {
          System.out.println(Math.abs(a+b)/2);
        } System.out.println(Math.abs(a+b)/2) System.out.println System.out System System.out Math.abs(a+b)/2 Math.abs(a+b) Math.abs Math a+b a b 2 {
          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) throws Exception {
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        int b = sc.nextInt();
        
        if ((a % 2 == 0 && b % 2 == 0) || (a % 2 != 0 && b % 2 != 0)) {
          System.out.println(Math.abs(a+b)/2);
        } else {
          System.out.println("IMPOSSIBLE");
        }

    }
} public class Main {
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        int b = sc.nextInt();
        
        if ((a % 2 == 0 && b % 2 == 0) || (a % 2 != 0 && b % 2 != 0)) {
          System.out.println(Math.abs(a+b)/2);
        } else {
          System.out.println("IMPOSSIBLE");
        }

    }
} Main
import java.util.*;

public class Main {
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        int b = sc.nextInt();
        
        if ((a % 2 == 0 && b % 2 == 0) || (a % 2 != 0 && b % 2 != 0)) {
          System.out.println(Math.abs(a+b)/2);
        } else {
          System.out.println("IMPOSSIBLE");
        }

    }
}
