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();
        double k = (a+b)/2d;
        if (Math.floor(k)==k)System.out.println((int)k);
        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);
        int a = sc.nextInt();
        int b = sc.nextInt();
        double k = (a+b)/2d;
        if (Math.floor(k)==k)System.out.println((int)k);
        else System.out.println("IMPOSSIBLE");
    }
} Main public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        int b = sc.nextInt();
        double k = (a+b)/2d;
        if (Math.floor(k)==k)System.out.println((int)k);
        else System.out.println("IMPOSSIBLE");
    } main {
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        int b = sc.nextInt();
        double k = (a+b)/2d;
        if (Math.floor(k)==k)System.out.println((int)k);
        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 double k = (a+b)/2d; k (a+b)/2d (a+b) a b 2d if (Math.floor(k)==k)System.out.println((int)k);
        else System.out.println("IMPOSSIBLE"); Math.floor(k)==k Math.floor(k) Math.floor Math k k System.out.println((int)k) System.out.println System.out System System.out (int)k 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);
        int a = sc.nextInt();
        int b = sc.nextInt();
        double k = (a+b)/2d;
        if (Math.floor(k)==k)System.out.println((int)k);
        else System.out.println("IMPOSSIBLE");
    }
} public class Main {
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        int b = sc.nextInt();
        double k = (a+b)/2d;
        if (Math.floor(k)==k)System.out.println((int)k);
        else System.out.println("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();
        double k = (a+b)/2d;
        if (Math.floor(k)==k)System.out.println((int)k);
        else System.out.println("IMPOSSIBLE");
    }
}
