import java.util.*;

public class Main {
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner(System.in);
        String line = sc.nextLine();
        String[] line1 = line.split(" ");
        int a = Integer.parseInt(line1[0]);
        int b = Integer.parseInt(line1[1]);
        int k;
        if((a + b) % 2 == 1)    System.out.println("IMPOSSIBLE");
        else{
            k = (a + b) / 2;
            System.out.println(k);
        }
    }
} import java.util.*; util.* java public class Main {
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner(System.in);
        String line = sc.nextLine();
        String[] line1 = line.split(" ");
        int a = Integer.parseInt(line1[0]);
        int b = Integer.parseInt(line1[1]);
        int k;
        if((a + b) % 2 == 1)    System.out.println("IMPOSSIBLE");
        else{
            k = (a + b) / 2;
            System.out.println(k);
        }
    }
} Main public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner(System.in);
        String line = sc.nextLine();
        String[] line1 = line.split(" ");
        int a = Integer.parseInt(line1[0]);
        int b = Integer.parseInt(line1[1]);
        int k;
        if((a + b) % 2 == 1)    System.out.println("IMPOSSIBLE");
        else{
            k = (a + b) / 2;
            System.out.println(k);
        }
    } main {
        Scanner sc = new Scanner(System.in);
        String line = sc.nextLine();
        String[] line1 = line.split(" ");
        int a = Integer.parseInt(line1[0]);
        int b = Integer.parseInt(line1[1]);
        int k;
        if((a + b) % 2 == 1)    System.out.println("IMPOSSIBLE");
        else{
            k = (a + b) / 2;
            System.out.println(k);
        }
    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) String line = sc.nextLine(); line sc.nextLine() sc.nextLine sc String[] line1 = line.split(" "); line1 line.split(" ") line.split line " " int a = Integer.parseInt(line1[0]); a Integer.parseInt(line1[0]) Integer.parseInt Integer line1[0] line1 0 int b = Integer.parseInt(line1[1]); b Integer.parseInt(line1[1]) Integer.parseInt Integer line1[1] line1 1 int k; k if((a + b) % 2 == 1)    System.out.println("IMPOSSIBLE");
        else{
            k = (a + b) / 2;
            System.out.println(k);
        } (a + b) % 2 == 1 (a + b) % 2 (a + b) a b 2 1 System.out.println("IMPOSSIBLE") System.out.println System.out System System.out "IMPOSSIBLE" {
            k = (a + b) / 2;
            System.out.println(k);
        } k = (a + b) / 2 k (a + b) / 2 (a + b) a b 2 System.out.println(k) System.out.println System.out System System.out k String[] args args public class Main {
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner(System.in);
        String line = sc.nextLine();
        String[] line1 = line.split(" ");
        int a = Integer.parseInt(line1[0]);
        int b = Integer.parseInt(line1[1]);
        int k;
        if((a + b) % 2 == 1)    System.out.println("IMPOSSIBLE");
        else{
            k = (a + b) / 2;
            System.out.println(k);
        }
    }
} public class Main {
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner(System.in);
        String line = sc.nextLine();
        String[] line1 = line.split(" ");
        int a = Integer.parseInt(line1[0]);
        int b = Integer.parseInt(line1[1]);
        int k;
        if((a + b) % 2 == 1)    System.out.println("IMPOSSIBLE");
        else{
            k = (a + b) / 2;
            System.out.println(k);
        }
    }
} Main
import java.util.*;

public class Main {
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner(System.in);
        String line = sc.nextLine();
        String[] line1 = line.split(" ");
        int a = Integer.parseInt(line1[0]);
        int b = Integer.parseInt(line1[1]);
        int k;
        if((a + b) % 2 == 1)    System.out.println("IMPOSSIBLE");
        else{
            k = (a + b) / 2;
            System.out.println(k);
        }
    }
}