import java.util.Scanner;

public class Main { 
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		String[] line = sc.nextLine().split(" ");
		long num1 = Long.parseLong(line[0]);
		long num2 = Long.parseLong(line[1]);

		if(num1%2 == num2%2) {
			long num3 = num1 + num2 ;
			System.out.println(num3/2);
		}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);
		String[] line = sc.nextLine().split(" ");
		long num1 = Long.parseLong(line[0]);
		long num2 = Long.parseLong(line[1]);

		if(num1%2 == num2%2) {
			long num3 = num1 + num2 ;
			System.out.println(num3/2);
		}else {
			System.out.println("IMPOSSIBLE");
		}
}

} Main public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		String[] line = sc.nextLine().split(" ");
		long num1 = Long.parseLong(line[0]);
		long num2 = Long.parseLong(line[1]);

		if(num1%2 == num2%2) {
			long num3 = num1 + num2 ;
			System.out.println(num3/2);
		}else {
			System.out.println("IMPOSSIBLE");
		}
} main {
		Scanner sc = new Scanner(System.in);
		String[] line = sc.nextLine().split(" ");
		long num1 = Long.parseLong(line[0]);
		long num2 = Long.parseLong(line[1]);

		if(num1%2 == num2%2) {
			long num3 = num1 + num2 ;
			System.out.println(num3/2);
		}else {
			System.out.println("IMPOSSIBLE");
		}
} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) String[] line = sc.nextLine().split(" "); line sc.nextLine().split(" ") sc.nextLine().split sc.nextLine() sc.nextLine sc " " long num1 = Long.parseLong(line[0]); num1 Long.parseLong(line[0]) Long.parseLong Long line[0] line 0 long num2 = Long.parseLong(line[1]); num2 Long.parseLong(line[1]) Long.parseLong Long line[1] line 1 if(num1%2 == num2%2) {
			long num3 = num1 + num2 ;
			System.out.println(num3/2);
		}else {
			System.out.println("IMPOSSIBLE");
		} num1%2 == num2%2 num1%2 num1 2 num2%2 num2 2 {
			long num3 = num1 + num2 ;
			System.out.println(num3/2);
		} long num3 = num1 + num2 ; num3 num1 + num2 num1 num2 System.out.println(num3/2) System.out.println System.out System System.out num3/2 num3 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);
		String[] line = sc.nextLine().split(" ");
		long num1 = Long.parseLong(line[0]);
		long num2 = Long.parseLong(line[1]);

		if(num1%2 == num2%2) {
			long num3 = num1 + num2 ;
			System.out.println(num3/2);
		}else {
			System.out.println("IMPOSSIBLE");
		}
}

} public class Main { 
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		String[] line = sc.nextLine().split(" ");
		long num1 = Long.parseLong(line[0]);
		long num2 = Long.parseLong(line[1]);

		if(num1%2 == num2%2) {
			long num3 = num1 + num2 ;
			System.out.println(num3/2);
		}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);
		String[] line = sc.nextLine().split(" ");
		long num1 = Long.parseLong(line[0]);
		long num2 = Long.parseLong(line[1]);

		if(num1%2 == num2%2) {
			long num3 = num1 + num2 ;
			System.out.println(num3/2);
		}else {
			System.out.println("IMPOSSIBLE");
		}
}

}