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();
		String ans = "";

		if (A % 2 != B % 2) {
			ans = "IMPOSSIBLE";
		} else {
			ans = String.valueOf((A + B) / 2);

		}

		System.out.println(ans);
		sc.close();

	}
} 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();
		String ans = "";

		if (A % 2 != B % 2) {
			ans = "IMPOSSIBLE";
		} else {
			ans = String.valueOf((A + B) / 2);

		}

		System.out.println(ans);
		sc.close();

	}
} Main public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int A = sc.nextInt();
		int B = sc.nextInt();
		String ans = "";

		if (A % 2 != B % 2) {
			ans = "IMPOSSIBLE";
		} else {
			ans = String.valueOf((A + B) / 2);

		}

		System.out.println(ans);
		sc.close();

	} main {
		Scanner sc = new Scanner(System.in);
		int A = sc.nextInt();
		int B = sc.nextInt();
		String ans = "";

		if (A % 2 != B % 2) {
			ans = "IMPOSSIBLE";
		} else {
			ans = String.valueOf((A + B) / 2);

		}

		System.out.println(ans);
		sc.close();

	} 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 String ans = ""; ans "" if (A % 2 != B % 2) {
			ans = "IMPOSSIBLE";
		} else {
			ans = String.valueOf((A + B) / 2);

		} A % 2 != B % 2 A % 2 A 2 B % 2 B 2 {
			ans = "IMPOSSIBLE";
		} ans = "IMPOSSIBLE" ans "IMPOSSIBLE" {
			ans = String.valueOf((A + B) / 2);

		} ans = String.valueOf((A + B) / 2) ans String.valueOf((A + B) / 2) String.valueOf String (A + B) / 2 (A + B) A B 2 System.out.println(ans) System.out.println System.out System System.out ans sc.close() sc.close sc 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();
		String ans = "";

		if (A % 2 != B % 2) {
			ans = "IMPOSSIBLE";
		} else {
			ans = String.valueOf((A + B) / 2);

		}

		System.out.println(ans);
		sc.close();

	}
} public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int A = sc.nextInt();
		int B = sc.nextInt();
		String ans = "";

		if (A % 2 != B % 2) {
			ans = "IMPOSSIBLE";
		} else {
			ans = String.valueOf((A + B) / 2);

		}

		System.out.println(ans);
		sc.close();

	}
} 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();
		String ans = "";

		if (A % 2 != B % 2) {
			ans = "IMPOSSIBLE";
		} else {
			ans = String.valueOf((A + B) / 2);

		}

		System.out.println(ans);
		sc.close();

	}
}
