import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        int b = sc.nextInt();
        int sum = a + b;
        if (sum % 2 == 1) {
            System.out.println("IMPOSSIBLE");
        } else {
            System.out.println(sum/2);
        }

    }
} import java.util.*; util.* java public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        int b = sc.nextInt();
        int sum = a + b;
        if (sum % 2 == 1) {
            System.out.println("IMPOSSIBLE");
        } else {
            System.out.println(sum/2);
        }

    }
} Main public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        int b = sc.nextInt();
        int sum = a + b;
        if (sum % 2 == 1) {
            System.out.println("IMPOSSIBLE");
        } else {
            System.out.println(sum/2);
        }

    } main {
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        int b = sc.nextInt();
        int sum = a + b;
        if (sum % 2 == 1) {
            System.out.println("IMPOSSIBLE");
        } else {
            System.out.println(sum/2);
        }

    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int a = sc.nextInt(); a sc.nextInt() sc.nextInt sc int b = sc.nextInt(); b sc.nextInt() sc.nextInt sc int sum = a + b; sum a + b a b if (sum % 2 == 1) {
            System.out.println("IMPOSSIBLE");
        } else {
            System.out.println(sum/2);
        } sum % 2 == 1 sum % 2 sum 2 1 {
            System.out.println("IMPOSSIBLE");
        } System.out.println("IMPOSSIBLE") System.out.println System.out System System.out "IMPOSSIBLE" {
            System.out.println(sum/2);
        } System.out.println(sum/2) System.out.println System.out System System.out sum/2 sum 2 String[] args args public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        int b = sc.nextInt();
        int sum = a + b;
        if (sum % 2 == 1) {
            System.out.println("IMPOSSIBLE");
        } else {
            System.out.println(sum/2);
        }

    }
} public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        int b = sc.nextInt();
        int sum = a + b;
        if (sum % 2 == 1) {
            System.out.println("IMPOSSIBLE");
        } else {
            System.out.println(sum/2);
        }

    }
} Main
import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        int b = sc.nextInt();
        int sum = a + b;
        if (sum % 2 == 1) {
            System.out.println("IMPOSSIBLE");
        } else {
            System.out.println(sum/2);
        }

    }
}
