
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(" ");
		long A = Long.parseLong(lines[0]);
		long B = Long.parseLong(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(" ");
		long A = Long.parseLong(lines[0]);
		long B = Long.parseLong(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(" ");
		long A = Long.parseLong(lines[0]);
		long B = Long.parseLong(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(" ");
		long A = Long.parseLong(lines[0]);
		long B = Long.parseLong(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 " " long A = Long.parseLong(lines[0]); A Long.parseLong(lines[0]) Long.parseLong Long lines[0] lines 0 long B = Long.parseLong(lines[1]); B Long.parseLong(lines[1]) Long.parseLong Long 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(" ");
		long A = Long.parseLong(lines[0]);
		long B = Long.parseLong(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(" ");
		long A = Long.parseLong(lines[0]);
		long B = Long.parseLong(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(" ");
		long A = Long.parseLong(lines[0]);
		long B = Long.parseLong(lines[1]);

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

}
