import java.util.Scanner;

public class Main {
    public static void main(String[]args){
        int a,b,sub1,sub2;
        Scanner in= new Scanner(System.in);
        a=in.nextInt(); b=in.nextInt();
        sub1=a+b;
        if(sub1%2==0)
            System.out.println(""+(sub1/2));
        else
            System.out.println("IMPOSSIBLE");
    }
} import java.util.Scanner; Scanner java.util public class Main {
    public static void main(String[]args){
        int a,b,sub1,sub2;
        Scanner in= new Scanner(System.in);
        a=in.nextInt(); b=in.nextInt();
        sub1=a+b;
        if(sub1%2==0)
            System.out.println(""+(sub1/2));
        else
            System.out.println("IMPOSSIBLE");
    }
} Main public static void main(String[]args){
        int a,b,sub1,sub2;
        Scanner in= new Scanner(System.in);
        a=in.nextInt(); b=in.nextInt();
        sub1=a+b;
        if(sub1%2==0)
            System.out.println(""+(sub1/2));
        else
            System.out.println("IMPOSSIBLE");
    } main {
        int a,b,sub1,sub2;
        Scanner in= new Scanner(System.in);
        a=in.nextInt(); b=in.nextInt();
        sub1=a+b;
        if(sub1%2==0)
            System.out.println(""+(sub1/2));
        else
            System.out.println("IMPOSSIBLE");
    } int a a b b sub1 sub1 sub2; sub2 Scanner in= new Scanner(System.in); in new Scanner(System.in) a=in.nextInt() a in.nextInt() in.nextInt in b=in.nextInt() b in.nextInt() in.nextInt in sub1=a+b sub1 a+b a b if(sub1%2==0)
            System.out.println(""+(sub1/2));
        else
            System.out.println("IMPOSSIBLE"); sub1%2==0 sub1%2 sub1 2 0 System.out.println(""+(sub1/2)) System.out.println System.out System System.out ""+(sub1/2) "" (sub1/2) sub1 2 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){
        int a,b,sub1,sub2;
        Scanner in= new Scanner(System.in);
        a=in.nextInt(); b=in.nextInt();
        sub1=a+b;
        if(sub1%2==0)
            System.out.println(""+(sub1/2));
        else
            System.out.println("IMPOSSIBLE");
    }
} public class Main {
    public static void main(String[]args){
        int a,b,sub1,sub2;
        Scanner in= new Scanner(System.in);
        a=in.nextInt(); b=in.nextInt();
        sub1=a+b;
        if(sub1%2==0)
            System.out.println(""+(sub1/2));
        else
            System.out.println("IMPOSSIBLE");
    }
} Main
import java.util.Scanner;

public class Main {
    public static void main(String[]args){
        int a,b,sub1,sub2;
        Scanner in= new Scanner(System.in);
        a=in.nextInt(); b=in.nextInt();
        sub1=a+b;
        if(sub1%2==0)
            System.out.println(""+(sub1/2));
        else
            System.out.println("IMPOSSIBLE");
    }
}