import java.util.*;

class Main{
	public static void main(String[] $){
		Scanner s=new Scanner(System.in);
		int a=s.nextInt(),b=s.nextInt();
		System.out.println((a+b)%2==0?(a+b)/2:"IMPOSSIBLE");
	}
} import java.util.*; util.* java class Main{
	public static void main(String[] $){
		Scanner s=new Scanner(System.in);
		int a=s.nextInt(),b=s.nextInt();
		System.out.println((a+b)%2==0?(a+b)/2:"IMPOSSIBLE");
	}
} Main public static void main(String[] $){
		Scanner s=new Scanner(System.in);
		int a=s.nextInt(),b=s.nextInt();
		System.out.println((a+b)%2==0?(a+b)/2:"IMPOSSIBLE");
	} main {
		Scanner s=new Scanner(System.in);
		int a=s.nextInt(),b=s.nextInt();
		System.out.println((a+b)%2==0?(a+b)/2:"IMPOSSIBLE");
	} Scanner s=new Scanner(System.in); s new Scanner(System.in) int a=s.nextInt() a s.nextInt() s.nextInt s b=s.nextInt(); b s.nextInt() s.nextInt s System.out.println((a+b)%2==0?(a+b)/2:"IMPOSSIBLE") System.out.println System.out System System.out (a+b)%2==0?(a+b)/2:"IMPOSSIBLE" (a+b)%2==0 (a+b)%2 (a+b) a b 2 0 (a+b)/2 (a+b) a b 2 "IMPOSSIBLE" String[] $ $
import java.util.*;

class Main{
	public static void main(String[] $){
		Scanner s=new Scanner(System.in);
		int a=s.nextInt(),b=s.nextInt();
		System.out.println((a+b)%2==0?(a+b)/2:"IMPOSSIBLE");
	}
}
