import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Comparator;
import java.util.HashMap;
import java.util.Map;
import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		Scanner in = new Scanner(new BufferedReader(new InputStreamReader(System.in)));
		int a = in.nextInt();
		int b = in.nextInt();
		long k = a + b;
		if (k % 2 == 0)
			System.out.println(k / 2);
		else
			System.out.println("IMPOSSIBLE");
		in.close();
	}
} import java.io.BufferedReader; BufferedReader java.io import java.io.InputStreamReader; InputStreamReader java.io import java.util.ArrayList; ArrayList java.util import java.util.Arrays; Arrays java.util import java.util.Comparator; Comparator java.util import java.util.HashMap; HashMap java.util import java.util.Map; Map java.util import java.util.Scanner; Scanner java.util public class Main {

	public static void main(String[] args) {
		Scanner in = new Scanner(new BufferedReader(new InputStreamReader(System.in)));
		int a = in.nextInt();
		int b = in.nextInt();
		long k = a + b;
		if (k % 2 == 0)
			System.out.println(k / 2);
		else
			System.out.println("IMPOSSIBLE");
		in.close();
	}
} Main public static void main(String[] args) {
		Scanner in = new Scanner(new BufferedReader(new InputStreamReader(System.in)));
		int a = in.nextInt();
		int b = in.nextInt();
		long k = a + b;
		if (k % 2 == 0)
			System.out.println(k / 2);
		else
			System.out.println("IMPOSSIBLE");
		in.close();
	} main {
		Scanner in = new Scanner(new BufferedReader(new InputStreamReader(System.in)));
		int a = in.nextInt();
		int b = in.nextInt();
		long k = a + b;
		if (k % 2 == 0)
			System.out.println(k / 2);
		else
			System.out.println("IMPOSSIBLE");
		in.close();
	} Scanner in = new Scanner(new BufferedReader(new InputStreamReader(System.in))); in new Scanner(new BufferedReader(new InputStreamReader(System.in))) int a = in.nextInt(); a in.nextInt() in.nextInt in int b = in.nextInt(); b in.nextInt() in.nextInt in long k = a + b; k a + b a b 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 System.out System System.out k / 2 k 2 System.out.println("IMPOSSIBLE") System.out.println System.out System System.out "IMPOSSIBLE" in.close() in.close in String[] args args public class Main {

	public static void main(String[] args) {
		Scanner in = new Scanner(new BufferedReader(new InputStreamReader(System.in)));
		int a = in.nextInt();
		int b = in.nextInt();
		long k = a + b;
		if (k % 2 == 0)
			System.out.println(k / 2);
		else
			System.out.println("IMPOSSIBLE");
		in.close();
	}
} public class Main {

	public static void main(String[] args) {
		Scanner in = new Scanner(new BufferedReader(new InputStreamReader(System.in)));
		int a = in.nextInt();
		int b = in.nextInt();
		long k = a + b;
		if (k % 2 == 0)
			System.out.println(k / 2);
		else
			System.out.println("IMPOSSIBLE");
		in.close();
	}
} Main
import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Comparator;
import java.util.HashMap;
import java.util.Map;
import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		Scanner in = new Scanner(new BufferedReader(new InputStreamReader(System.in)));
		int a = in.nextInt();
		int b = in.nextInt();
		long k = a + b;
		if (k % 2 == 0)
			System.out.println(k / 2);
		else
			System.out.println("IMPOSSIBLE");
		in.close();
	}
}
