import java.util.*;

public class Main {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        long a = sc.nextLong();
        long b = sc.nextInt();
        double a1=a/2.0;
        double b1=b/2.0;
        int sum= (int) (a1+b1);
        if ( Math.abs(a-sum) == Math.abs(b-sum)) {
            System.out.println(sum);
        }
        else System.out.println("IMPOSSIBLE");
    }
} import java.util.*; util.* java public class Main {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        long a = sc.nextLong();
        long b = sc.nextInt();
        double a1=a/2.0;
        double b1=b/2.0;
        int sum= (int) (a1+b1);
        if ( Math.abs(a-sum) == Math.abs(b-sum)) {
            System.out.println(sum);
        }
        else System.out.println("IMPOSSIBLE");
    }
} Main public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        long a = sc.nextLong();
        long b = sc.nextInt();
        double a1=a/2.0;
        double b1=b/2.0;
        int sum= (int) (a1+b1);
        if ( Math.abs(a-sum) == Math.abs(b-sum)) {
            System.out.println(sum);
        }
        else System.out.println("IMPOSSIBLE");
    } main {
        Scanner sc = new Scanner(System.in);
        long a = sc.nextLong();
        long b = sc.nextInt();
        double a1=a/2.0;
        double b1=b/2.0;
        int sum= (int) (a1+b1);
        if ( Math.abs(a-sum) == Math.abs(b-sum)) {
            System.out.println(sum);
        }
        else System.out.println("IMPOSSIBLE");
    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) long a = sc.nextLong(); a sc.nextLong() sc.nextLong sc long b = sc.nextInt(); b sc.nextInt() sc.nextInt sc double a1=a/2.0; a1 a/2.0 a 2.0 double b1=b/2.0; b1 b/2.0 b 2.0 int sum= (int) (a1+b1); sum (int) (a1+b1) a1 b1 if ( Math.abs(a-sum) == Math.abs(b-sum)) {
            System.out.println(sum);
        }
        else System.out.println("IMPOSSIBLE"); Math.abs(a-sum) == Math.abs(b-sum) Math.abs(a-sum) Math.abs Math a-sum a sum Math.abs(b-sum) Math.abs Math b-sum b sum {
            System.out.println(sum);
        } System.out.println(sum) System.out.println System.out System System.out sum 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) {
        Scanner sc = new Scanner(System.in);
        long a = sc.nextLong();
        long b = sc.nextInt();
        double a1=a/2.0;
        double b1=b/2.0;
        int sum= (int) (a1+b1);
        if ( Math.abs(a-sum) == Math.abs(b-sum)) {
            System.out.println(sum);
        }
        else System.out.println("IMPOSSIBLE");
    }
} public class Main {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        long a = sc.nextLong();
        long b = sc.nextInt();
        double a1=a/2.0;
        double b1=b/2.0;
        int sum= (int) (a1+b1);
        if ( Math.abs(a-sum) == Math.abs(b-sum)) {
            System.out.println(sum);
        }
        else System.out.println("IMPOSSIBLE");
    }
} Main
import java.util.*;

public class Main {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        long a = sc.nextLong();
        long b = sc.nextInt();
        double a1=a/2.0;
        double b1=b/2.0;
        int sum= (int) (a1+b1);
        if ( Math.abs(a-sum) == Math.abs(b-sum)) {
            System.out.println(sum);
        }
        else System.out.println("IMPOSSIBLE");
    }
}
