import java.util.Scanner;

public class Main {

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

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

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

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

public class Main {

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

