import java.util.*;
import static java.lang.Math.*;

public class Main {
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);

		// 入力
		int a = sc.nextInt();
		int b = sc.nextInt();

		// 計算
		int k = a + b;
		String result = "No";

		// 出力
		if(k%2 == 0){
		   System.out.println(k/2);
		}else{
		   System.out.println("IMPOSSIBLE");
		}
	}
	
} import java.util.*; util.* java import static java.lang.Math.*; import static java.lang.Math.*; import static java.lang.Math.*; public class Main {
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);

		// 入力
		int a = sc.nextInt();
		int b = sc.nextInt();

		// 計算
		int k = a + b;
		String result = "No";

		// 出力
		if(k%2 == 0){
		   System.out.println(k/2);
		}else{
		   System.out.println("IMPOSSIBLE");
		}
	}
	
} Main public static void main(String[] args){
		Scanner sc = new Scanner(System.in);

		// 入力
		int a = sc.nextInt();
		int b = sc.nextInt();

		// 計算
		int k = a + b;
		String result = "No";

		// 出力
		if(k%2 == 0){
		   System.out.println(k/2);
		}else{
		   System.out.println("IMPOSSIBLE");
		}
	} main {
		Scanner sc = new Scanner(System.in);

		// 入力
		int a = sc.nextInt();
		int b = sc.nextInt();

		// 計算
		int k = a + b;
		String result = "No";

		// 出力
		if(k%2 == 0){
		   System.out.println(k/2);
		}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 int k = a + b; k a + b a b String result = "No"; result "No" if(k%2 == 0){
		   System.out.println(k/2);
		}else{
		   System.out.println("IMPOSSIBLE");
		} k%2 == 0 k%2 k 2 0 {
		   System.out.println(k/2);
		} System.out.println(k/2) System.out.println System.out System System.out k/2 k 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);

		// 入力
		int a = sc.nextInt();
		int b = sc.nextInt();

		// 計算
		int k = a + b;
		String result = "No";

		// 出力
		if(k%2 == 0){
		   System.out.println(k/2);
		}else{
		   System.out.println("IMPOSSIBLE");
		}
	}
	
} public class Main {
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);

		// 入力
		int a = sc.nextInt();
		int b = sc.nextInt();

		// 計算
		int k = a + b;
		String result = "No";

		// 出力
		if(k%2 == 0){
		   System.out.println(k/2);
		}else{
		   System.out.println("IMPOSSIBLE");
		}
	}
	
} Main
import java.util.*;
import static java.lang.Math.*;

public class Main {
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);

		// 入力
		int a = sc.nextInt();
		int b = sc.nextInt();

		// 計算
		int k = a + b;
		String result = "No";

		// 出力
		if(k%2 == 0){
		   System.out.println(k/2);
		}else{
		   System.out.println("IMPOSSIBLE");
		}
	}
	
}
