

import java.util.InputMismatchException;
import java.util.Scanner;

public class Main{
	public static void main(String[] args) {
		try {
			Scanner scanner = new Scanner(System.in);

			int a = scanner.nextInt();
			int b = scanner.nextInt();

			if ((a + b) % 2 == 0) {
				int answer = (a + b) / 2;
				System.out.println(answer);
			}else{
				System.out.println("IMPOSSIBLE");
			}
			scanner.close();

		} catch (InputMismatchException e) {
			// TODO: handle exception
			System.out.println("整数を入力してください");
		}
	}

}
 import java.util.InputMismatchException; InputMismatchException java.util import java.util.Scanner; Scanner java.util public class Main{
	public static void main(String[] args) {
		try {
			Scanner scanner = new Scanner(System.in);

			int a = scanner.nextInt();
			int b = scanner.nextInt();

			if ((a + b) % 2 == 0) {
				int answer = (a + b) / 2;
				System.out.println(answer);
			}else{
				System.out.println("IMPOSSIBLE");
			}
			scanner.close();

		} catch (InputMismatchException e) {
			// TODO: handle exception
			System.out.println("整数を入力してください");
		}
	}

} Main public static void main(String[] args) {
		try {
			Scanner scanner = new Scanner(System.in);

			int a = scanner.nextInt();
			int b = scanner.nextInt();

			if ((a + b) % 2 == 0) {
				int answer = (a + b) / 2;
				System.out.println(answer);
			}else{
				System.out.println("IMPOSSIBLE");
			}
			scanner.close();

		} catch (InputMismatchException e) {
			// TODO: handle exception
			System.out.println("整数を入力してください");
		}
	} main {
		try {
			Scanner scanner = new Scanner(System.in);

			int a = scanner.nextInt();
			int b = scanner.nextInt();

			if ((a + b) % 2 == 0) {
				int answer = (a + b) / 2;
				System.out.println(answer);
			}else{
				System.out.println("IMPOSSIBLE");
			}
			scanner.close();

		} catch (InputMismatchException e) {
			// TODO: handle exception
			System.out.println("整数を入力してください");
		}
	} try {
			Scanner scanner = new Scanner(System.in);

			int a = scanner.nextInt();
			int b = scanner.nextInt();

			if ((a + b) % 2 == 0) {
				int answer = (a + b) / 2;
				System.out.println(answer);
			}else{
				System.out.println("IMPOSSIBLE");
			}
			scanner.close();

		} catch (InputMismatchException e) {
			// TODO: handle exception
			System.out.println("整数を入力してください");
		} catch (InputMismatchException e) {
			// TODO: handle exception
			System.out.println("整数を入力してください");
		} InputMismatchException e {
			// TODO: handle exception
			System.out.println("整数を入力してください");
		} System.out.println("整数を入力してください") System.out.println System.out System System.out "整数を入力してください" {
			Scanner scanner = new Scanner(System.in);

			int a = scanner.nextInt();
			int b = scanner.nextInt();

			if ((a + b) % 2 == 0) {
				int answer = (a + b) / 2;
				System.out.println(answer);
			}else{
				System.out.println("IMPOSSIBLE");
			}
			scanner.close();

		} Scanner scanner = new Scanner(System.in); scanner new Scanner(System.in) int a = scanner.nextInt(); a scanner.nextInt() scanner.nextInt scanner int b = scanner.nextInt(); b scanner.nextInt() scanner.nextInt scanner if ((a + b) % 2 == 0) {
				int answer = (a + b) / 2;
				System.out.println(answer);
			}else{
				System.out.println("IMPOSSIBLE");
			} (a + b) % 2 == 0 (a + b) % 2 (a + b) a b 2 0 {
				int answer = (a + b) / 2;
				System.out.println(answer);
			} int answer = (a + b) / 2; answer (a + b) / 2 (a + b) a b 2 System.out.println(answer) System.out.println System.out System System.out answer {
				System.out.println("IMPOSSIBLE");
			} System.out.println("IMPOSSIBLE") System.out.println System.out System System.out "IMPOSSIBLE" scanner.close() scanner.close scanner String[] args args public class Main{
	public static void main(String[] args) {
		try {
			Scanner scanner = new Scanner(System.in);

			int a = scanner.nextInt();
			int b = scanner.nextInt();

			if ((a + b) % 2 == 0) {
				int answer = (a + b) / 2;
				System.out.println(answer);
			}else{
				System.out.println("IMPOSSIBLE");
			}
			scanner.close();

		} catch (InputMismatchException e) {
			// TODO: handle exception
			System.out.println("整数を入力してください");
		}
	}

} public class Main{
	public static void main(String[] args) {
		try {
			Scanner scanner = new Scanner(System.in);

			int a = scanner.nextInt();
			int b = scanner.nextInt();

			if ((a + b) % 2 == 0) {
				int answer = (a + b) / 2;
				System.out.println(answer);
			}else{
				System.out.println("IMPOSSIBLE");
			}
			scanner.close();

		} catch (InputMismatchException e) {
			// TODO: handle exception
			System.out.println("整数を入力してください");
		}
	}

} Main


import java.util.InputMismatchException;
import java.util.Scanner;

public class Main{
	public static void main(String[] args) {
		try {
			Scanner scanner = new Scanner(System.in);

			int a = scanner.nextInt();
			int b = scanner.nextInt();

			if ((a + b) % 2 == 0) {
				int answer = (a + b) / 2;
				System.out.println(answer);
			}else{
				System.out.println("IMPOSSIBLE");
			}
			scanner.close();

		} catch (InputMismatchException e) {
			// TODO: handle exception
			System.out.println("整数を入力してください");
		}
	}

}

