import java.util.Scanner;

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

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