import java.util.*;

class Main{
	public static void main(String[] args){
	Scanner scan = new Scanner(System.in);
	int a = scan.nextInt();
	int b = scan.nextInt();
	int c = a+b;
	boolean isEven = (a+b)%2==0;

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

	if(isEven){
		System.out.println((a+b)/2);
	}else{
		System.out.println("IMPOSSIBLE");
	}
	}
} Main public static void main(String[] args){
	Scanner scan = new Scanner(System.in);
	int a = scan.nextInt();
	int b = scan.nextInt();
	int c = a+b;
	boolean isEven = (a+b)%2==0;

	if(isEven){
		System.out.println((a+b)/2);
	}else{
		System.out.println("IMPOSSIBLE");
	}
	} main {
	Scanner scan = new Scanner(System.in);
	int a = scan.nextInt();
	int b = scan.nextInt();
	int c = a+b;
	boolean isEven = (a+b)%2==0;

	if(isEven){
		System.out.println((a+b)/2);
	}else{
		System.out.println("IMPOSSIBLE");
	}
	} Scanner scan = new Scanner(System.in); scan new Scanner(System.in) int a = scan.nextInt(); a scan.nextInt() scan.nextInt scan int b = scan.nextInt(); b scan.nextInt() scan.nextInt scan int c = a+b; c a+b a b boolean isEven = (a+b)%2==0; isEven (a+b)%2==0 (a+b)%2 (a+b) a b 2 0 if(isEven){
		System.out.println((a+b)/2);
	}else{
		System.out.println("IMPOSSIBLE");
	} isEven {
		System.out.println((a+b)/2);
	} System.out.println((a+b)/2) System.out.println System.out System System.out (a+b)/2 (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
import java.util.*;

class Main{
	public static void main(String[] args){
	Scanner scan = new Scanner(System.in);
	int a = scan.nextInt();
	int b = scan.nextInt();
	int c = a+b;
	boolean isEven = (a+b)%2==0;

	if(isEven){
		System.out.println((a+b)/2);
	}else{
		System.out.println("IMPOSSIBLE");
	}
	}
}
