import java.util.Scanner;
public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		long A = sc.nextLong();
		long B = sc.nextLong();
      	long diff = A + B;
      	if (diff % 2 == 1) {
          System.out.println("IMPOSSIBLE");
        } else {
		  System.out.println(diff / 2);
        }
	}
} import java.util.Scanner; Scanner java.util public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		long A = sc.nextLong();
		long B = sc.nextLong();
      	long diff = A + B;
      	if (diff % 2 == 1) {
          System.out.println("IMPOSSIBLE");
        } else {
		  System.out.println(diff / 2);
        }
	}
} Main public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		long A = sc.nextLong();
		long B = sc.nextLong();
      	long diff = A + B;
      	if (diff % 2 == 1) {
          System.out.println("IMPOSSIBLE");
        } else {
		  System.out.println(diff / 2);
        }
	} main {
		Scanner sc = new Scanner(System.in);
		long A = sc.nextLong();
		long B = sc.nextLong();
      	long diff = A + B;
      	if (diff % 2 == 1) {
          System.out.println("IMPOSSIBLE");
        } else {
		  System.out.println(diff / 2);
        }
	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) long A = sc.nextLong(); A sc.nextLong() sc.nextLong sc long B = sc.nextLong(); B sc.nextLong() sc.nextLong sc long diff = A + B; diff A + B A B if (diff % 2 == 1) {
          System.out.println("IMPOSSIBLE");
        } else {
		  System.out.println(diff / 2);
        } diff % 2 == 1 diff % 2 diff 2 1 {
          System.out.println("IMPOSSIBLE");
        } System.out.println("IMPOSSIBLE") System.out.println System.out System System.out "IMPOSSIBLE" {
		  System.out.println(diff / 2);
        } System.out.println(diff / 2) System.out.println System.out System System.out diff / 2 diff 2 String[] args args public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		long A = sc.nextLong();
		long B = sc.nextLong();
      	long diff = A + B;
      	if (diff % 2 == 1) {
          System.out.println("IMPOSSIBLE");
        } else {
		  System.out.println(diff / 2);
        }
	}
} public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		long A = sc.nextLong();
		long B = sc.nextLong();
      	long diff = A + B;
      	if (diff % 2 == 1) {
          System.out.println("IMPOSSIBLE");
        } else {
		  System.out.println(diff / 2);
        }
	}
} Main
import java.util.Scanner;
public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		long A = sc.nextLong();
		long B = sc.nextLong();
      	long diff = A + B;
      	if (diff % 2 == 1) {
          System.out.println("IMPOSSIBLE");
        } else {
		  System.out.println(diff / 2);
        }
	}
}