import java.io.PrintWriter;
import java.util.*;

public class Main {
		public static void main(String[] args) {
			Scanner stdin = new Scanner(System.in);
			PrintWriter out = new PrintWriter(System.out);
			
			int A = Integer.parseInt(stdin.next());
			int B = Integer.parseInt(stdin.next());
			
			if(A%2 != B%2) {
				out.print("IMPOSSIBLE");
			}
			else {
				out.print((A+B)/2);
			}
			
			out.flush();
	}
}

 import java.io.PrintWriter; PrintWriter java.io import java.util.*; util.* java public class Main {
		public static void main(String[] args) {
			Scanner stdin = new Scanner(System.in);
			PrintWriter out = new PrintWriter(System.out);
			
			int A = Integer.parseInt(stdin.next());
			int B = Integer.parseInt(stdin.next());
			
			if(A%2 != B%2) {
				out.print("IMPOSSIBLE");
			}
			else {
				out.print((A+B)/2);
			}
			
			out.flush();
	}
} Main public static void main(String[] args) {
			Scanner stdin = new Scanner(System.in);
			PrintWriter out = new PrintWriter(System.out);
			
			int A = Integer.parseInt(stdin.next());
			int B = Integer.parseInt(stdin.next());
			
			if(A%2 != B%2) {
				out.print("IMPOSSIBLE");
			}
			else {
				out.print((A+B)/2);
			}
			
			out.flush();
	} main {
			Scanner stdin = new Scanner(System.in);
			PrintWriter out = new PrintWriter(System.out);
			
			int A = Integer.parseInt(stdin.next());
			int B = Integer.parseInt(stdin.next());
			
			if(A%2 != B%2) {
				out.print("IMPOSSIBLE");
			}
			else {
				out.print((A+B)/2);
			}
			
			out.flush();
	} Scanner stdin = new Scanner(System.in); stdin new Scanner(System.in) PrintWriter out = new PrintWriter(System.out); out new PrintWriter(System.out) int A = Integer.parseInt(stdin.next()); A Integer.parseInt(stdin.next()) Integer.parseInt Integer stdin.next() stdin.next stdin int B = Integer.parseInt(stdin.next()); B Integer.parseInt(stdin.next()) Integer.parseInt Integer stdin.next() stdin.next stdin if(A%2 != B%2) {
				out.print("IMPOSSIBLE");
			}
			else {
				out.print((A+B)/2);
			} A%2 != B%2 A%2 A 2 B%2 B 2 {
				out.print("IMPOSSIBLE");
			} out.print("IMPOSSIBLE") out.print out "IMPOSSIBLE" {
				out.print((A+B)/2);
			} out.print((A+B)/2) out.print out (A+B)/2 (A+B) A B 2 out.flush() out.flush out String[] args args public class Main {
		public static void main(String[] args) {
			Scanner stdin = new Scanner(System.in);
			PrintWriter out = new PrintWriter(System.out);
			
			int A = Integer.parseInt(stdin.next());
			int B = Integer.parseInt(stdin.next());
			
			if(A%2 != B%2) {
				out.print("IMPOSSIBLE");
			}
			else {
				out.print((A+B)/2);
			}
			
			out.flush();
	}
} public class Main {
		public static void main(String[] args) {
			Scanner stdin = new Scanner(System.in);
			PrintWriter out = new PrintWriter(System.out);
			
			int A = Integer.parseInt(stdin.next());
			int B = Integer.parseInt(stdin.next());
			
			if(A%2 != B%2) {
				out.print("IMPOSSIBLE");
			}
			else {
				out.print((A+B)/2);
			}
			
			out.flush();
	}
} Main
import java.io.PrintWriter;
import java.util.*;

public class Main {
		public static void main(String[] args) {
			Scanner stdin = new Scanner(System.in);
			PrintWriter out = new PrintWriter(System.out);
			
			int A = Integer.parseInt(stdin.next());
			int B = Integer.parseInt(stdin.next());
			
			if(A%2 != B%2) {
				out.print("IMPOSSIBLE");
			}
			else {
				out.print((A+B)/2);
			}
			
			out.flush();
	}
}


