import java.util.*;
public class Main {
	public static void main(String[] args){
		// 整数の入力
		Scanner sc = new Scanner(System.in);
		// 整数の入力
		int a = sc.nextInt();
		int b = sc.nextInt();
		// 絶対値の差分を取る10
		int c = Math.abs(a - b);

		// 偶数の場合はNG
		if(c % 2 != 0) {
			System.out.println("IMPOSSIBLE");
		}else {
			int k = (a + b) / 2;
			System.out.println(k);
		}
	}
} import java.util.*; util.* java public class Main {
	public static void main(String[] args){
		// 整数の入力
		Scanner sc = new Scanner(System.in);
		// 整数の入力
		int a = sc.nextInt();
		int b = sc.nextInt();
		// 絶対値の差分を取る10
		int c = Math.abs(a - b);

		// 偶数の場合はNG
		if(c % 2 != 0) {
			System.out.println("IMPOSSIBLE");
		}else {
			int k = (a + b) / 2;
			System.out.println(k);
		}
	}
} Main public static void main(String[] args){
		// 整数の入力
		Scanner sc = new Scanner(System.in);
		// 整数の入力
		int a = sc.nextInt();
		int b = sc.nextInt();
		// 絶対値の差分を取る10
		int c = Math.abs(a - b);

		// 偶数の場合はNG
		if(c % 2 != 0) {
			System.out.println("IMPOSSIBLE");
		}else {
			int k = (a + b) / 2;
			System.out.println(k);
		}
	} main {
		// 整数の入力
		Scanner sc = new Scanner(System.in);
		// 整数の入力
		int a = sc.nextInt();
		int b = sc.nextInt();
		// 絶対値の差分を取る10
		int c = Math.abs(a - b);

		// 偶数の場合はNG
		if(c % 2 != 0) {
			System.out.println("IMPOSSIBLE");
		}else {
			int k = (a + b) / 2;
			System.out.println(k);
		}
	} 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 int c = Math.abs(a - b); c Math.abs(a - b) Math.abs Math a - b a b if(c % 2 != 0) {
			System.out.println("IMPOSSIBLE");
		}else {
			int k = (a + b) / 2;
			System.out.println(k);
		} c % 2 != 0 c % 2 c 2 0 {
			System.out.println("IMPOSSIBLE");
		} System.out.println("IMPOSSIBLE") System.out.println System.out System System.out "IMPOSSIBLE" {
			int k = (a + b) / 2;
			System.out.println(k);
		} int k = (a + b) / 2; k (a + b) / 2 (a + b) a b 2 System.out.println(k) System.out.println System.out System System.out k String[] args args public class Main {
	public static void main(String[] args){
		// 整数の入力
		Scanner sc = new Scanner(System.in);
		// 整数の入力
		int a = sc.nextInt();
		int b = sc.nextInt();
		// 絶対値の差分を取る10
		int c = Math.abs(a - b);

		// 偶数の場合はNG
		if(c % 2 != 0) {
			System.out.println("IMPOSSIBLE");
		}else {
			int k = (a + b) / 2;
			System.out.println(k);
		}
	}
} public class Main {
	public static void main(String[] args){
		// 整数の入力
		Scanner sc = new Scanner(System.in);
		// 整数の入力
		int a = sc.nextInt();
		int b = sc.nextInt();
		// 絶対値の差分を取る10
		int c = Math.abs(a - b);

		// 偶数の場合はNG
		if(c % 2 != 0) {
			System.out.println("IMPOSSIBLE");
		}else {
			int k = (a + b) / 2;
			System.out.println(k);
		}
	}
} Main
import java.util.*;
public class Main {
	public static void main(String[] args){
		// 整数の入力
		Scanner sc = new Scanner(System.in);
		// 整数の入力
		int a = sc.nextInt();
		int b = sc.nextInt();
		// 絶対値の差分を取る10
		int c = Math.abs(a - b);

		// 偶数の場合はNG
		if(c % 2 != 0) {
			System.out.println("IMPOSSIBLE");
		}else {
			int k = (a + b) / 2;
			System.out.println(k);
		}
	}
}
