import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);

        int a = input.nextInt();
        int b = input.nextInt();

        if((a+b)%2 == 0 ) {
            System.out.println((a+b)/2);
        }else {
            System.out.println("IMPOSSIBLE");
        }

    }
}
