import java.util.Scanner;

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

public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int A = sc.nextInt();
		int B = sc.nextInt();
		sc.close();
		String IMPOSSIBLE = "IMPOSSIBLE";
		if(A % 2 != B % 2) System.out.println(IMPOSSIBLE);
		else System.out.println((A + B) / 2);
	}
}
