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


	}

}