import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
	    int A = sc.nextInt();
	    int B = sc.nextInt();
	    
	    double doubleK = (double)(A + B) / 2;
	    int intK = (A + B) / 2;
	    
	    if (doubleK == intK) {
	    	System.out.println(intK);
	    } else {
	    	System.out.println("IMPOSSIBLE");
	    }
	}	
}	 import java.util.Scanner; Scanner java.util public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
	    int A = sc.nextInt();
	    int B = sc.nextInt();
	    
	    double doubleK = (double)(A + B) / 2;
	    int intK = (A + B) / 2;
	    
	    if (doubleK == intK) {
	    	System.out.println(intK);
	    } else {
	    	System.out.println("IMPOSSIBLE");
	    }
	}	
} Main public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
	    int A = sc.nextInt();
	    int B = sc.nextInt();
	    
	    double doubleK = (double)(A + B) / 2;
	    int intK = (A + B) / 2;
	    
	    if (doubleK == intK) {
	    	System.out.println(intK);
	    } else {
	    	System.out.println("IMPOSSIBLE");
	    }
	} main {
		Scanner sc = new Scanner(System.in);
	    int A = sc.nextInt();
	    int B = sc.nextInt();
	    
	    double doubleK = (double)(A + B) / 2;
	    int intK = (A + B) / 2;
	    
	    if (doubleK == intK) {
	    	System.out.println(intK);
	    } 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 double doubleK = (double)(A + B) / 2; doubleK (double)(A + B) / 2 (double)(A + B) A B 2 int intK = (A + B) / 2; intK (A + B) / 2 (A + B) A B 2 if (doubleK == intK) {
	    	System.out.println(intK);
	    } else {
	    	System.out.println("IMPOSSIBLE");
	    } doubleK == intK doubleK intK {
	    	System.out.println(intK);
	    } System.out.println(intK) System.out.println System.out System System.out intK {
	    	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);
	    int A = sc.nextInt();
	    int B = sc.nextInt();
	    
	    double doubleK = (double)(A + B) / 2;
	    int intK = (A + B) / 2;
	    
	    if (doubleK == intK) {
	    	System.out.println(intK);
	    } else {
	    	System.out.println("IMPOSSIBLE");
	    }
	}	
} public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
	    int A = sc.nextInt();
	    int B = sc.nextInt();
	    
	    double doubleK = (double)(A + B) / 2;
	    int intK = (A + B) / 2;
	    
	    if (doubleK == intK) {
	    	System.out.println(intK);
	    } else {
	    	System.out.println("IMPOSSIBLE");
	    }
	}	
} Main
import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
	    int A = sc.nextInt();
	    int B = sc.nextInt();
	    
	    double doubleK = (double)(A + B) / 2;
	    int intK = (A + B) / 2;
	    
	    if (doubleK == intK) {
	    	System.out.println(intK);
	    } else {
	    	System.out.println("IMPOSSIBLE");
	    }
	}	
}	