import java.util.*;
public class Main {
	public static void main(String args[]) {
		Scanner sc = new Scanner(System.in);
		double A = sc.nextDouble();
		double B = sc.nextDouble();
		if(((double)Math.abs(A - B)) % 2 == 0) {
			System.out.println((int)(Math.max(A, B) - Math.abs(A - B) / 2));
		} else {
			System.out.println("IMPOSSIBLE");
		}
	}
} import java.util.*; util.* java public class Main {
	public static void main(String args[]) {
		Scanner sc = new Scanner(System.in);
		double A = sc.nextDouble();
		double B = sc.nextDouble();
		if(((double)Math.abs(A - B)) % 2 == 0) {
			System.out.println((int)(Math.max(A, B) - Math.abs(A - B) / 2));
		} else {
			System.out.println("IMPOSSIBLE");
		}
	}
} Main public static void main(String args[]) {
		Scanner sc = new Scanner(System.in);
		double A = sc.nextDouble();
		double B = sc.nextDouble();
		if(((double)Math.abs(A - B)) % 2 == 0) {
			System.out.println((int)(Math.max(A, B) - Math.abs(A - B) / 2));
		} else {
			System.out.println("IMPOSSIBLE");
		}
	} main {
		Scanner sc = new Scanner(System.in);
		double A = sc.nextDouble();
		double B = sc.nextDouble();
		if(((double)Math.abs(A - B)) % 2 == 0) {
			System.out.println((int)(Math.max(A, B) - Math.abs(A - B) / 2));
		} else {
			System.out.println("IMPOSSIBLE");
		}
	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) double A = sc.nextDouble(); A sc.nextDouble() sc.nextDouble sc double B = sc.nextDouble(); B sc.nextDouble() sc.nextDouble sc if(((double)Math.abs(A - B)) % 2 == 0) {
			System.out.println((int)(Math.max(A, B) - Math.abs(A - B) / 2));
		} else {
			System.out.println("IMPOSSIBLE");
		} ((double)Math.abs(A - B)) % 2 == 0 ((double)Math.abs(A - B)) % 2 ((double)Math.abs(A - B)) Math.abs Math A - B A B 2 0 {
			System.out.println((int)(Math.max(A, B) - Math.abs(A - B) / 2));
		} System.out.println((int)(Math.max(A, B) - Math.abs(A - B) / 2)) System.out.println System.out System System.out (int)(Math.max(A, B) - Math.abs(A - B) / 2) Math.max(A, B) Math.max Math A B Math.abs(A - B) / 2 Math.abs(A - B) Math.abs Math A - B A B 2 {
			System.out.println("IMPOSSIBLE");
		} 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[]) {
		Scanner sc = new Scanner(System.in);
		double A = sc.nextDouble();
		double B = sc.nextDouble();
		if(((double)Math.abs(A - B)) % 2 == 0) {
			System.out.println((int)(Math.max(A, B) - Math.abs(A - B) / 2));
		} else {
			System.out.println("IMPOSSIBLE");
		}
	}
} public class Main {
	public static void main(String args[]) {
		Scanner sc = new Scanner(System.in);
		double A = sc.nextDouble();
		double B = sc.nextDouble();
		if(((double)Math.abs(A - B)) % 2 == 0) {
			System.out.println((int)(Math.max(A, B) - Math.abs(A - B) / 2));
		} else {
			System.out.println("IMPOSSIBLE");
		}
	}
} Main
import java.util.*;
public class Main {
	public static void main(String args[]) {
		Scanner sc = new Scanner(System.in);
		double A = sc.nextDouble();
		double B = sc.nextDouble();
		if(((double)Math.abs(A - B)) % 2 == 0) {
			System.out.println((int)(Math.max(A, B) - Math.abs(A - B) / 2));
		} else {
			System.out.println("IMPOSSIBLE");
		}
	}
}
