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

		double A = in.nextDouble();
		double B = in.nextDouble();
		
		double C = Math.abs((A+B)/2);
		if(C==(int)C) {
			System.out.println((int)C);
		}
		else {
			System.out.println("IMPOSSIBLE");
		}
	}

} import java.util.*; util.* java public class Main {
	public static void main(String[] args) {
	
		Scanner in = new Scanner(System.in);

		double A = in.nextDouble();
		double B = in.nextDouble();
		
		double C = Math.abs((A+B)/2);
		if(C==(int)C) {
			System.out.println((int)C);
		}
		else {
			System.out.println("IMPOSSIBLE");
		}
	}

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

		double A = in.nextDouble();
		double B = in.nextDouble();
		
		double C = Math.abs((A+B)/2);
		if(C==(int)C) {
			System.out.println((int)C);
		}
		else {
			System.out.println("IMPOSSIBLE");
		}
	} main {
	
		Scanner in = new Scanner(System.in);

		double A = in.nextDouble();
		double B = in.nextDouble();
		
		double C = Math.abs((A+B)/2);
		if(C==(int)C) {
			System.out.println((int)C);
		}
		else {
			System.out.println("IMPOSSIBLE");
		}
	} Scanner in = new Scanner(System.in); in new Scanner(System.in) double A = in.nextDouble(); A in.nextDouble() in.nextDouble in double B = in.nextDouble(); B in.nextDouble() in.nextDouble in double C = Math.abs((A+B)/2); C Math.abs((A+B)/2) Math.abs Math (A+B)/2 (A+B) A B 2 if(C==(int)C) {
			System.out.println((int)C);
		}
		else {
			System.out.println("IMPOSSIBLE");
		} C==(int)C C (int)C {
			System.out.println((int)C);
		} System.out.println((int)C) System.out.println System.out System System.out (int)C {
			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 in = new Scanner(System.in);

		double A = in.nextDouble();
		double B = in.nextDouble();
		
		double C = Math.abs((A+B)/2);
		if(C==(int)C) {
			System.out.println((int)C);
		}
		else {
			System.out.println("IMPOSSIBLE");
		}
	}

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

		double A = in.nextDouble();
		double B = in.nextDouble();
		
		double C = Math.abs((A+B)/2);
		if(C==(int)C) {
			System.out.println((int)C);
		}
		else {
			System.out.println("IMPOSSIBLE");
		}
	}

} Main
import java.util.*;
public class Main {
	public static void main(String[] args) {
	
		Scanner in = new Scanner(System.in);

		double A = in.nextDouble();
		double B = in.nextDouble();
		
		double C = Math.abs((A+B)/2);
		if(C==(int)C) {
			System.out.println((int)C);
		}
		else {
			System.out.println("IMPOSSIBLE");
		}
	}

}
