import java.util.*;
public class Main {
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
    
		// 整数の入力
		int a = sc.nextInt();
		int b = sc.nextInt();
        int max = Math.max(a, b);
        int min = Math.min(a, b);
        int sum = a + b;
    
		// 出力
		System.out.println((sum) % 2 == 0 ? sum / 2 : "IMPOSSIBLE");
	}
} import java.util.*; util.* java public class Main {
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
    
		// 整数の入力
		int a = sc.nextInt();
		int b = sc.nextInt();
        int max = Math.max(a, b);
        int min = Math.min(a, b);
        int sum = a + b;
    
		// 出力
		System.out.println((sum) % 2 == 0 ? sum / 2 : "IMPOSSIBLE");
	}
} Main public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
    
		// 整数の入力
		int a = sc.nextInt();
		int b = sc.nextInt();
        int max = Math.max(a, b);
        int min = Math.min(a, b);
        int sum = a + b;
    
		// 出力
		System.out.println((sum) % 2 == 0 ? sum / 2 : "IMPOSSIBLE");
	} main {
		Scanner sc = new Scanner(System.in);
    
		// 整数の入力
		int a = sc.nextInt();
		int b = sc.nextInt();
        int max = Math.max(a, b);
        int min = Math.min(a, b);
        int sum = a + b;
    
		// 出力
		System.out.println((sum) % 2 == 0 ? sum / 2 : "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 max = Math.max(a, b); max Math.max(a, b) Math.max Math a b int min = Math.min(a, b); min Math.min(a, b) Math.min Math a b int sum = a + b; sum a + b a b System.out.println((sum) % 2 == 0 ? sum / 2 : "IMPOSSIBLE") System.out.println System.out System System.out (sum) % 2 == 0 ? sum / 2 : "IMPOSSIBLE" (sum) % 2 == 0 (sum) % 2 (sum) 2 0 sum / 2 sum 2 "IMPOSSIBLE" String[] args args public class Main {
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
    
		// 整数の入力
		int a = sc.nextInt();
		int b = sc.nextInt();
        int max = Math.max(a, b);
        int min = Math.min(a, b);
        int sum = a + b;
    
		// 出力
		System.out.println((sum) % 2 == 0 ? sum / 2 : "IMPOSSIBLE");
	}
} public class Main {
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
    
		// 整数の入力
		int a = sc.nextInt();
		int b = sc.nextInt();
        int max = Math.max(a, b);
        int min = Math.min(a, b);
        int sum = a + b;
    
		// 出力
		System.out.println((sum) % 2 == 0 ? sum / 2 : "IMPOSSIBLE");
	}
} Main
import java.util.*;
public class Main {
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
    
		// 整数の入力
		int a = sc.nextInt();
		int b = sc.nextInt();
        int max = Math.max(a, b);
        int min = Math.min(a, b);
        int sum = a + b;
    
		// 出力
		System.out.println((sum) % 2 == 0 ? sum / 2 : "IMPOSSIBLE");
	}
}