import java.util.*;

public class Main{
	public static void main(String args[]) {
		
		Scanner sc = new Scanner(System.in);
		long x = sc.nextLong() + sc.nextLong();
		sc.close();
		System.out.println((x % 2 == 0)?x / 2:"IMPOSSIBLE");
	}
} import java.util.*; util.* java public class Main{
	public static void main(String args[]) {
		
		Scanner sc = new Scanner(System.in);
		long x = sc.nextLong() + sc.nextLong();
		sc.close();
		System.out.println((x % 2 == 0)?x / 2:"IMPOSSIBLE");
	}
} Main public static void main(String args[]) {
		
		Scanner sc = new Scanner(System.in);
		long x = sc.nextLong() + sc.nextLong();
		sc.close();
		System.out.println((x % 2 == 0)?x / 2:"IMPOSSIBLE");
	} main {
		
		Scanner sc = new Scanner(System.in);
		long x = sc.nextLong() + sc.nextLong();
		sc.close();
		System.out.println((x % 2 == 0)?x / 2:"IMPOSSIBLE");
	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) long x = sc.nextLong() + sc.nextLong(); x sc.nextLong() + sc.nextLong() sc.nextLong() sc.nextLong sc sc.nextLong() sc.nextLong sc sc.close() sc.close sc System.out.println((x % 2 == 0)?x / 2:"IMPOSSIBLE") System.out.println System.out System System.out (x % 2 == 0)?x / 2:"IMPOSSIBLE" (x % 2 == 0) x % 2 x 2 0 x / 2 x 2 "IMPOSSIBLE" String args[] args public class Main{
	public static void main(String args[]) {
		
		Scanner sc = new Scanner(System.in);
		long x = sc.nextLong() + sc.nextLong();
		sc.close();
		System.out.println((x % 2 == 0)?x / 2:"IMPOSSIBLE");
	}
} public class Main{
	public static void main(String args[]) {
		
		Scanner sc = new Scanner(System.in);
		long x = sc.nextLong() + sc.nextLong();
		sc.close();
		System.out.println((x % 2 == 0)?x / 2:"IMPOSSIBLE");
	}
} Main
import java.util.*;

public class Main{
	public static void main(String args[]) {
		
		Scanner sc = new Scanner(System.in);
		long x = sc.nextLong() + sc.nextLong();
		sc.close();
		System.out.println((x % 2 == 0)?x / 2:"IMPOSSIBLE");
	}
}