import java.util.*;

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

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