import java.util.*;
public class Main {

	public static void main(String[] args) {
		Scanner x = new Scanner(System.in);
		long A = x.nextLong();
		long B = x.nextLong();
		long K=0;
		
		while(true) {
			long c = Math.abs(A-K);
			long d = Math.abs(B-K);
			if(c == d) {
				System.out.println(K);
				break;
			}
			else if(c != d) {
				System.out.println("IMPOSSIBLLE");
				break;

			}
			K++;
			
		}

	}

}
