import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		//int n = sc.nextInt();
		//String[][] array = new String[100][500];
		//List<String> list= new ArrayList<String>(Arrays.asList(s.split("")));
		List<Integer> list=new ArrayList<Integer>();
		int a = sc.nextInt();
		int b = sc.nextInt();

		int c = Math.abs(a-b);
		if(c%2==0) {
			if(a<b) {
				System.out.println(a+c/2);
			}
			else {
				System.out.println(a-c/2);
			}

		}
		else {
			System.out.println("IMPOSSIBLE");
		}
	}

} import java.util.ArrayList; ArrayList java.util import java.util.List; List java.util import java.util.Scanner; Scanner java.util public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		//int n = sc.nextInt();
		//String[][] array = new String[100][500];
		//List<String> list= new ArrayList<String>(Arrays.asList(s.split("")));
		List<Integer> list=new ArrayList<Integer>();
		int a = sc.nextInt();
		int b = sc.nextInt();

		int c = Math.abs(a-b);
		if(c%2==0) {
			if(a<b) {
				System.out.println(a+c/2);
			}
			else {
				System.out.println(a-c/2);
			}

		}
		else {
			System.out.println("IMPOSSIBLE");
		}
	}

} Main public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		//int n = sc.nextInt();
		//String[][] array = new String[100][500];
		//List<String> list= new ArrayList<String>(Arrays.asList(s.split("")));
		List<Integer> list=new ArrayList<Integer>();
		int a = sc.nextInt();
		int b = sc.nextInt();

		int c = Math.abs(a-b);
		if(c%2==0) {
			if(a<b) {
				System.out.println(a+c/2);
			}
			else {
				System.out.println(a-c/2);
			}

		}
		else {
			System.out.println("IMPOSSIBLE");
		}
	} main {
		Scanner sc = new Scanner(System.in);
		//int n = sc.nextInt();
		//String[][] array = new String[100][500];
		//List<String> list= new ArrayList<String>(Arrays.asList(s.split("")));
		List<Integer> list=new ArrayList<Integer>();
		int a = sc.nextInt();
		int b = sc.nextInt();

		int c = Math.abs(a-b);
		if(c%2==0) {
			if(a<b) {
				System.out.println(a+c/2);
			}
			else {
				System.out.println(a-c/2);
			}

		}
		else {
			System.out.println("IMPOSSIBLE");
		}
	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) List<Integer> list=new ArrayList<Integer>(); list new ArrayList<Integer>() 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) {
			if(a<b) {
				System.out.println(a+c/2);
			}
			else {
				System.out.println(a-c/2);
			}

		}
		else {
			System.out.println("IMPOSSIBLE");
		} c%2==0 c%2 c 2 0 {
			if(a<b) {
				System.out.println(a+c/2);
			}
			else {
				System.out.println(a-c/2);
			}

		} if(a<b) {
				System.out.println(a+c/2);
			}
			else {
				System.out.println(a-c/2);
			} a<b a b {
				System.out.println(a+c/2);
			} System.out.println(a+c/2) System.out.println System.out System System.out a+c/2 a c/2 c 2 {
				System.out.println(a-c/2);
			} System.out.println(a-c/2) System.out.println System.out System System.out a-c/2 a c/2 c 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);
		//int n = sc.nextInt();
		//String[][] array = new String[100][500];
		//List<String> list= new ArrayList<String>(Arrays.asList(s.split("")));
		List<Integer> list=new ArrayList<Integer>();
		int a = sc.nextInt();
		int b = sc.nextInt();

		int c = Math.abs(a-b);
		if(c%2==0) {
			if(a<b) {
				System.out.println(a+c/2);
			}
			else {
				System.out.println(a-c/2);
			}

		}
		else {
			System.out.println("IMPOSSIBLE");
		}
	}

} public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		//int n = sc.nextInt();
		//String[][] array = new String[100][500];
		//List<String> list= new ArrayList<String>(Arrays.asList(s.split("")));
		List<Integer> list=new ArrayList<Integer>();
		int a = sc.nextInt();
		int b = sc.nextInt();

		int c = Math.abs(a-b);
		if(c%2==0) {
			if(a<b) {
				System.out.println(a+c/2);
			}
			else {
				System.out.println(a-c/2);
			}

		}
		else {
			System.out.println("IMPOSSIBLE");
		}
	}

} Main
import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		//int n = sc.nextInt();
		//String[][] array = new String[100][500];
		//List<String> list= new ArrayList<String>(Arrays.asList(s.split("")));
		List<Integer> list=new ArrayList<Integer>();
		int a = sc.nextInt();
		int b = sc.nextInt();

		int c = Math.abs(a-b);
		if(c%2==0) {
			if(a<b) {
				System.out.println(a+c/2);
			}
			else {
				System.out.println(a-c/2);
			}

		}
		else {
			System.out.println("IMPOSSIBLE");
		}
	}

}