import java.io.BufferedReader;
import java.io.InputStreamReader;

public class Main {
    public static void main(String args[]) throws Exception {
	BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
	String strArray[] = br.readLine().split(" ");
	int A = Integer.parseInt(strArray[0]);
	int B = Integer.parseInt(strArray[1]);
	if ((A + B) % 2 == 0)
	    System.out.println((A + B) / 2);
	else
	    System.out.println("IMPOSSIBLE");
    }
} import java.io.BufferedReader; BufferedReader java.io import java.io.InputStreamReader; InputStreamReader java.io public class Main {
    public static void main(String args[]) throws Exception {
	BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
	String strArray[] = br.readLine().split(" ");
	int A = Integer.parseInt(strArray[0]);
	int B = Integer.parseInt(strArray[1]);
	if ((A + B) % 2 == 0)
	    System.out.println((A + B) / 2);
	else
	    System.out.println("IMPOSSIBLE");
    }
} Main public static void main(String args[]) throws Exception {
	BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
	String strArray[] = br.readLine().split(" ");
	int A = Integer.parseInt(strArray[0]);
	int B = Integer.parseInt(strArray[1]);
	if ((A + B) % 2 == 0)
	    System.out.println((A + B) / 2);
	else
	    System.out.println("IMPOSSIBLE");
    } main {
	BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
	String strArray[] = br.readLine().split(" ");
	int A = Integer.parseInt(strArray[0]);
	int B = Integer.parseInt(strArray[1]);
	if ((A + B) % 2 == 0)
	    System.out.println((A + B) / 2);
	else
	    System.out.println("IMPOSSIBLE");
    } BufferedReader br = new BufferedReader(new InputStreamReader(System.in)); br new BufferedReader(new InputStreamReader(System.in)) String strArray[] = br.readLine().split(" "); strArray br.readLine().split(" ") br.readLine().split br.readLine() br.readLine br " " int A = Integer.parseInt(strArray[0]); A Integer.parseInt(strArray[0]) Integer.parseInt Integer strArray[0] strArray 0 int B = Integer.parseInt(strArray[1]); B Integer.parseInt(strArray[1]) Integer.parseInt Integer strArray[1] strArray 1 if ((A + B) % 2 == 0)
	    System.out.println((A + B) / 2);
	else
	    System.out.println("IMPOSSIBLE"); (A + B) % 2 == 0 (A + B) % 2 (A + B) A B 2 0 System.out.println((A + B) / 2) System.out.println System.out System System.out (A + B) / 2 (A + B) A B 2 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[]) throws Exception {
	BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
	String strArray[] = br.readLine().split(" ");
	int A = Integer.parseInt(strArray[0]);
	int B = Integer.parseInt(strArray[1]);
	if ((A + B) % 2 == 0)
	    System.out.println((A + B) / 2);
	else
	    System.out.println("IMPOSSIBLE");
    }
} public class Main {
    public static void main(String args[]) throws Exception {
	BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
	String strArray[] = br.readLine().split(" ");
	int A = Integer.parseInt(strArray[0]);
	int B = Integer.parseInt(strArray[1]);
	if ((A + B) % 2 == 0)
	    System.out.println((A + B) / 2);
	else
	    System.out.println("IMPOSSIBLE");
    }
} Main
import java.io.BufferedReader;
import java.io.InputStreamReader;

public class Main {
    public static void main(String args[]) throws Exception {
	BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
	String strArray[] = br.readLine().split(" ");
	int A = Integer.parseInt(strArray[0]);
	int B = Integer.parseInt(strArray[1]);
	if ((A + B) % 2 == 0)
	    System.out.println((A + B) / 2);
	else
	    System.out.println("IMPOSSIBLE");
    }
}
