import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		// TODO 自動生成されたメソッド・スタブ
		Scanner sc = new Scanner(System.in);
		String line = sc.nextLine();
		String[] lines = line.split(" ");

		int A = Integer.parseInt(lines[0]);
		int B = Integer.parseInt(lines[1]);

		if( (A+B) % 2 == 0) {
			System.out.println((A+B) / 2 );
		}
		else {
			System.out.println("IMPOSSIBLE");
		}
	}

} import java.util.Scanner; Scanner java.util public class Main {

	public static void main(String[] args) {
		// TODO 自動生成されたメソッド・スタブ
		Scanner sc = new Scanner(System.in);
		String line = sc.nextLine();
		String[] lines = line.split(" ");

		int A = Integer.parseInt(lines[0]);
		int B = Integer.parseInt(lines[1]);

		if( (A+B) % 2 == 0) {
			System.out.println((A+B) / 2 );
		}
		else {
			System.out.println("IMPOSSIBLE");
		}
	}

} Main public static void main(String[] args) {
		// TODO 自動生成されたメソッド・スタブ
		Scanner sc = new Scanner(System.in);
		String line = sc.nextLine();
		String[] lines = line.split(" ");

		int A = Integer.parseInt(lines[0]);
		int B = Integer.parseInt(lines[1]);

		if( (A+B) % 2 == 0) {
			System.out.println((A+B) / 2 );
		}
		else {
			System.out.println("IMPOSSIBLE");
		}
	} main {
		// TODO 自動生成されたメソッド・スタブ
		Scanner sc = new Scanner(System.in);
		String line = sc.nextLine();
		String[] lines = line.split(" ");

		int A = Integer.parseInt(lines[0]);
		int B = Integer.parseInt(lines[1]);

		if( (A+B) % 2 == 0) {
			System.out.println((A+B) / 2 );
		}
		else {
			System.out.println("IMPOSSIBLE");
		}
	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) String line = sc.nextLine(); line sc.nextLine() sc.nextLine sc String[] lines = line.split(" "); lines line.split(" ") line.split line " " int A = Integer.parseInt(lines[0]); A Integer.parseInt(lines[0]) Integer.parseInt Integer lines[0] lines 0 int B = Integer.parseInt(lines[1]); B Integer.parseInt(lines[1]) Integer.parseInt Integer lines[1] lines 1 if( (A+B) % 2 == 0) {
			System.out.println((A+B) / 2 );
		}
		else {
			System.out.println("IMPOSSIBLE");
		} (A+B) % 2 == 0 (A+B) % 2 (A+B) A B 2 0 {
			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 public class Main {

	public static void main(String[] args) {
		// TODO 自動生成されたメソッド・スタブ
		Scanner sc = new Scanner(System.in);
		String line = sc.nextLine();
		String[] lines = line.split(" ");

		int A = Integer.parseInt(lines[0]);
		int B = Integer.parseInt(lines[1]);

		if( (A+B) % 2 == 0) {
			System.out.println((A+B) / 2 );
		}
		else {
			System.out.println("IMPOSSIBLE");
		}
	}

} public class Main {

	public static void main(String[] args) {
		// TODO 自動生成されたメソッド・スタブ
		Scanner sc = new Scanner(System.in);
		String line = sc.nextLine();
		String[] lines = line.split(" ");

		int A = Integer.parseInt(lines[0]);
		int B = Integer.parseInt(lines[1]);

		if( (A+B) % 2 == 0) {
			System.out.println((A+B) / 2 );
		}
		else {
			System.out.println("IMPOSSIBLE");
		}
	}

} Main
import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		// TODO 自動生成されたメソッド・スタブ
		Scanner sc = new Scanner(System.in);
		String line = sc.nextLine();
		String[] lines = line.split(" ");

		int A = Integer.parseInt(lines[0]);
		int B = Integer.parseInt(lines[1]);

		if( (A+B) % 2 == 0) {
			System.out.println((A+B) / 2 );
		}
		else {
			System.out.println("IMPOSSIBLE");
		}
	}

}