import java.util.*;

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

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        int b = sc.nextInt();
        int ans = Math.abs(a+b);
        System.out.println(ans % 2 == 0? ans/2: "IMPOSSIBLE");
    }
}