import java.util.*;
//import static java.lang.Integer.*;
import static java.lang.Long.*;
//import static java.lang.Math.*;
import static java.lang.System.*;

public class Main {
	public static void main(String[] args) {
		int i,j;
		Scanner sc = new Scanner(System.in);
		long a = parseLong(sc.next());
		long b = parseLong(sc.next());
		sc.close();
		long k;
		k = a+b;
		if((k&1)==0)out.println(k/2);
		else out.println("IMPOSSIBLE");
	}
} import java.util.*; util.* java import static java.lang.Long.*; import static java.lang.Long.*; import static java.lang.Long.*; import static java.lang.System.*; import static java.lang.System.*; import static java.lang.System.*; public class Main {
	public static void main(String[] args) {
		int i,j;
		Scanner sc = new Scanner(System.in);
		long a = parseLong(sc.next());
		long b = parseLong(sc.next());
		sc.close();
		long k;
		k = a+b;
		if((k&1)==0)out.println(k/2);
		else out.println("IMPOSSIBLE");
	}
} Main public static void main(String[] args) {
		int i,j;
		Scanner sc = new Scanner(System.in);
		long a = parseLong(sc.next());
		long b = parseLong(sc.next());
		sc.close();
		long k;
		k = a+b;
		if((k&1)==0)out.println(k/2);
		else out.println("IMPOSSIBLE");
	} main {
		int i,j;
		Scanner sc = new Scanner(System.in);
		long a = parseLong(sc.next());
		long b = parseLong(sc.next());
		sc.close();
		long k;
		k = a+b;
		if((k&1)==0)out.println(k/2);
		else out.println("IMPOSSIBLE");
	} int i i j; j Scanner sc = new Scanner(System.in); sc new Scanner(System.in) long a = parseLong(sc.next()); a parseLong(sc.next()) parseLong sc.next() sc.next sc long b = parseLong(sc.next()); b parseLong(sc.next()) parseLong sc.next() sc.next sc sc.close() sc.close sc long k; k k = a+b k a+b a b if((k&1)==0)out.println(k/2);
		else out.println("IMPOSSIBLE"); (k&1)==0 (k&1) k 1 0 out.println(k/2) out.println out k/2 k 2 out.println("IMPOSSIBLE") out.println out "IMPOSSIBLE" String[] args args public class Main {
	public static void main(String[] args) {
		int i,j;
		Scanner sc = new Scanner(System.in);
		long a = parseLong(sc.next());
		long b = parseLong(sc.next());
		sc.close();
		long k;
		k = a+b;
		if((k&1)==0)out.println(k/2);
		else out.println("IMPOSSIBLE");
	}
} public class Main {
	public static void main(String[] args) {
		int i,j;
		Scanner sc = new Scanner(System.in);
		long a = parseLong(sc.next());
		long b = parseLong(sc.next());
		sc.close();
		long k;
		k = a+b;
		if((k&1)==0)out.println(k/2);
		else out.println("IMPOSSIBLE");
	}
} Main
import java.util.*;
//import static java.lang.Integer.*;
import static java.lang.Long.*;
//import static java.lang.Math.*;
import static java.lang.System.*;

public class Main {
	public static void main(String[] args) {
		int i,j;
		Scanner sc = new Scanner(System.in);
		long a = parseLong(sc.next());
		long b = parseLong(sc.next());
		sc.close();
		long k;
		k = a+b;
		if((k&1)==0)out.println(k/2);
		else out.println("IMPOSSIBLE");
	}
}
