import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		StringBuilder sb = new StringBuilder();
		int a = sc.nextInt();
		int b = sc.nextInt();
		if ((a - b) % 2 == 0) {
			System.out.println(a - (a - b) / 2);
		} else {
			System.out.println("IMPOSSIBLE");
		}
	}
} import java.util.Scanner; Scanner java.util public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		StringBuilder sb = new StringBuilder();
		int a = sc.nextInt();
		int b = sc.nextInt();
		if ((a - b) % 2 == 0) {
			System.out.println(a - (a - b) / 2);
		} else {
			System.out.println("IMPOSSIBLE");
		}
	}
} Main public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		StringBuilder sb = new StringBuilder();
		int a = sc.nextInt();
		int b = sc.nextInt();
		if ((a - b) % 2 == 0) {
			System.out.println(a - (a - b) / 2);
		} else {
			System.out.println("IMPOSSIBLE");
		}
	} main {
		Scanner sc = new Scanner(System.in);
		StringBuilder sb = new StringBuilder();
		int a = sc.nextInt();
		int b = sc.nextInt();
		if ((a - b) % 2 == 0) {
			System.out.println(a - (a - b) / 2);
		} else {
			System.out.println("IMPOSSIBLE");
		}
	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) StringBuilder sb = new StringBuilder(); sb new StringBuilder() int a = sc.nextInt(); a sc.nextInt() sc.nextInt sc int b = sc.nextInt(); b sc.nextInt() sc.nextInt sc if ((a - b) % 2 == 0) {
			System.out.println(a - (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 - (a - b) / 2);
		} System.out.println(a - (a - b) / 2) System.out.println System.out System System.out a - (a - b) / 2 a (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) {
		Scanner sc = new Scanner(System.in);
		StringBuilder sb = new StringBuilder();
		int a = sc.nextInt();
		int b = sc.nextInt();
		if ((a - b) % 2 == 0) {
			System.out.println(a - (a - b) / 2);
		} else {
			System.out.println("IMPOSSIBLE");
		}
	}
} public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		StringBuilder sb = new StringBuilder();
		int a = sc.nextInt();
		int b = sc.nextInt();
		if ((a - b) % 2 == 0) {
			System.out.println(a - (a - b) / 2);
		} else {
			System.out.println("IMPOSSIBLE");
		}
	}
} Main
import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		StringBuilder sb = new StringBuilder();
		int a = sc.nextInt();
		int b = sc.nextInt();
		if ((a - b) % 2 == 0) {
			System.out.println(a - (a - b) / 2);
		} else {
			System.out.println("IMPOSSIBLE");
		}
	}
}