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