import java.util.Scanner;

// https://atcoder.jp/contests/abc135/tasks/abc135_a
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();
		sc.close();

		int A = Math.min(a, b);
		int B = Math.max(a, b);

		if((A+B)%2==0) {
			int K = (A+B)/2;

			if((A-K)<=0 && (B-K)>=0) {
				System.out.println(K);
			} else {
				System.out.println("IMPOSSIBLE");
			}
		} 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();
		sc.close();

		int A = Math.min(a, b);
		int B = Math.max(a, b);

		if((A+B)%2==0) {
			int K = (A+B)/2;

			if((A-K)<=0 && (B-K)>=0) {
				System.out.println(K);
			} else {
				System.out.println("IMPOSSIBLE");
			}
		} 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();
		sc.close();

		int A = Math.min(a, b);
		int B = Math.max(a, b);

		if((A+B)%2==0) {
			int K = (A+B)/2;

			if((A-K)<=0 && (B-K)>=0) {
				System.out.println(K);
			} else {
				System.out.println("IMPOSSIBLE");
			}
		} 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();
		sc.close();

		int A = Math.min(a, b);
		int B = Math.max(a, b);

		if((A+B)%2==0) {
			int K = (A+B)/2;

			if((A-K)<=0 && (B-K)>=0) {
				System.out.println(K);
			} else {
				System.out.println("IMPOSSIBLE");
			}
		} 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 sc.close() sc.close sc int A = Math.min(a, b); A Math.min(a, b) Math.min Math a b int B = Math.max(a, b); B Math.max(a, b) Math.max Math a b if((A+B)%2==0) {
			int K = (A+B)/2;

			if((A-K)<=0 && (B-K)>=0) {
				System.out.println(K);
			} else {
				System.out.println("IMPOSSIBLE");
			}
		} else {
			System.out.println("IMPOSSIBLE");
		} (A+B)%2==0 (A+B)%2 (A+B) A B 2 0 {
			int K = (A+B)/2;

			if((A-K)<=0 && (B-K)>=0) {
				System.out.println(K);
			} else {
				System.out.println("IMPOSSIBLE");
			}
		} int K = (A+B)/2; K (A+B)/2 (A+B) A B 2 if((A-K)<=0 && (B-K)>=0) {
				System.out.println(K);
			} else {
				System.out.println("IMPOSSIBLE");
			} (A-K)<=0 && (B-K)>=0 (A-K)<=0 (A-K) A K 0 (B-K)>=0 (B-K) B K 0 {
				System.out.println(K);
			} System.out.println(K) System.out.println System.out System System.out K {
				System.out.println("IMPOSSIBLE");
			} System.out.println("IMPOSSIBLE") System.out.println System.out System System.out "IMPOSSIBLE" {
			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();
		sc.close();

		int A = Math.min(a, b);
		int B = Math.max(a, b);

		if((A+B)%2==0) {
			int K = (A+B)/2;

			if((A-K)<=0 && (B-K)>=0) {
				System.out.println(K);
			} else {
				System.out.println("IMPOSSIBLE");
			}
		} 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();
		sc.close();

		int A = Math.min(a, b);
		int B = Math.max(a, b);

		if((A+B)%2==0) {
			int K = (A+B)/2;

			if((A-K)<=0 && (B-K)>=0) {
				System.out.println(K);
			} else {
				System.out.println("IMPOSSIBLE");
			}
		} else {
			System.out.println("IMPOSSIBLE");
		}





	}

} Main
import java.util.Scanner;

// https://atcoder.jp/contests/abc135/tasks/abc135_a
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();
		sc.close();

		int A = Math.min(a, b);
		int B = Math.max(a, b);

		if((A+B)%2==0) {
			int K = (A+B)/2;

			if((A-K)<=0 && (B-K)>=0) {
				System.out.println(K);
			} else {
				System.out.println("IMPOSSIBLE");
			}
		} else {
			System.out.println("IMPOSSIBLE");
		}





	}

}
