import java.util.*;

public class Main {
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner(System.in);
        int n = Integer.parseInt(sc.next());
        int y = Integer.parseInt(sc.next());
        long z = n+y;
        System.out.println((n+y)%2==0?(n+y)/2:"IMPOSSIBLE");
        
}
} import java.util.*; util.* java public class Main {
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner(System.in);
        int n = Integer.parseInt(sc.next());
        int y = Integer.parseInt(sc.next());
        long z = n+y;
        System.out.println((n+y)%2==0?(n+y)/2:"IMPOSSIBLE");
        
}
} Main public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner(System.in);
        int n = Integer.parseInt(sc.next());
        int y = Integer.parseInt(sc.next());
        long z = n+y;
        System.out.println((n+y)%2==0?(n+y)/2:"IMPOSSIBLE");
        
} main {
        Scanner sc = new Scanner(System.in);
        int n = Integer.parseInt(sc.next());
        int y = Integer.parseInt(sc.next());
        long z = n+y;
        System.out.println((n+y)%2==0?(n+y)/2:"IMPOSSIBLE");
        
} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int n = Integer.parseInt(sc.next()); n Integer.parseInt(sc.next()) Integer.parseInt Integer sc.next() sc.next sc int y = Integer.parseInt(sc.next()); y Integer.parseInt(sc.next()) Integer.parseInt Integer sc.next() sc.next sc long z = n+y; z n+y n y System.out.println((n+y)%2==0?(n+y)/2:"IMPOSSIBLE") System.out.println System.out System System.out (n+y)%2==0?(n+y)/2:"IMPOSSIBLE" (n+y)%2==0 (n+y)%2 (n+y) n y 2 0 (n+y)/2 (n+y) n y 2 "IMPOSSIBLE" String[] args args public class Main {
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner(System.in);
        int n = Integer.parseInt(sc.next());
        int y = Integer.parseInt(sc.next());
        long z = n+y;
        System.out.println((n+y)%2==0?(n+y)/2:"IMPOSSIBLE");
        
}
} public class Main {
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner(System.in);
        int n = Integer.parseInt(sc.next());
        int y = Integer.parseInt(sc.next());
        long z = n+y;
        System.out.println((n+y)%2==0?(n+y)/2:"IMPOSSIBLE");
        
}
} Main
import java.util.*;

public class Main {
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner(System.in);
        int n = Integer.parseInt(sc.next());
        int y = Integer.parseInt(sc.next());
        long z = n+y;
        System.out.println((n+y)%2==0?(n+y)/2:"IMPOSSIBLE");
        
}
}