import java.util.Scanner;

public class Main {
	public static void solution(int x, int y) {
		if((x+y)%2==0) {
			System.out.println((x+y)/2);
		}
		else {
			System.out.println("IMPOSSIBLE");
		}
	}
		
	
	public static void main(String args []) {
		Scanner obj = new Scanner(System.in);
		int x = obj.nextInt();
		int y = obj.nextInt();
		solution(x,y);
	}
} import java.util.Scanner; Scanner java.util public class Main {
	public static void solution(int x, int y) {
		if((x+y)%2==0) {
			System.out.println((x+y)/2);
		}
		else {
			System.out.println("IMPOSSIBLE");
		}
	}
		
	
	public static void main(String args []) {
		Scanner obj = new Scanner(System.in);
		int x = obj.nextInt();
		int y = obj.nextInt();
		solution(x,y);
	}
} Main public static void solution(int x, int y) {
		if((x+y)%2==0) {
			System.out.println((x+y)/2);
		}
		else {
			System.out.println("IMPOSSIBLE");
		}
	} solution {
		if((x+y)%2==0) {
			System.out.println((x+y)/2);
		}
		else {
			System.out.println("IMPOSSIBLE");
		}
	} if((x+y)%2==0) {
			System.out.println((x+y)/2);
		}
		else {
			System.out.println("IMPOSSIBLE");
		} (x+y)%2==0 (x+y)%2 (x+y) x y 2 0 {
			System.out.println((x+y)/2);
		} System.out.println((x+y)/2) System.out.println System.out System System.out (x+y)/2 (x+y) x y 2 {
			System.out.println("IMPOSSIBLE");
		} System.out.println("IMPOSSIBLE") System.out.println System.out System System.out "IMPOSSIBLE" int x x int y y public static void main(String args []) {
		Scanner obj = new Scanner(System.in);
		int x = obj.nextInt();
		int y = obj.nextInt();
		solution(x,y);
	} main {
		Scanner obj = new Scanner(System.in);
		int x = obj.nextInt();
		int y = obj.nextInt();
		solution(x,y);
	} Scanner obj = new Scanner(System.in); obj new Scanner(System.in) int x = obj.nextInt(); x obj.nextInt() obj.nextInt obj int y = obj.nextInt(); y obj.nextInt() obj.nextInt obj solution(x,y) solution x y String args [] args public class Main {
	public static void solution(int x, int y) {
		if((x+y)%2==0) {
			System.out.println((x+y)/2);
		}
		else {
			System.out.println("IMPOSSIBLE");
		}
	}
		
	
	public static void main(String args []) {
		Scanner obj = new Scanner(System.in);
		int x = obj.nextInt();
		int y = obj.nextInt();
		solution(x,y);
	}
} public class Main {
	public static void solution(int x, int y) {
		if((x+y)%2==0) {
			System.out.println((x+y)/2);
		}
		else {
			System.out.println("IMPOSSIBLE");
		}
	}
		
	
	public static void main(String args []) {
		Scanner obj = new Scanner(System.in);
		int x = obj.nextInt();
		int y = obj.nextInt();
		solution(x,y);
	}
} Main
import java.util.Scanner;

public class Main {
	public static void solution(int x, int y) {
		if((x+y)%2==0) {
			System.out.println((x+y)/2);
		}
		else {
			System.out.println("IMPOSSIBLE");
		}
	}
		
	
	public static void main(String args []) {
		Scanner obj = new Scanner(System.in);
		int x = obj.nextInt();
		int y = obj.nextInt();
		solution(x,y);
	}
}