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();

        if((A-B)/2==(double)(A-B)/2){
            if(A>=B){
                System.out.println(A-(A-B)/2);
            }
            if(A<B){
                System.out.println(B-(B-A)/2);
            }
        }
        else{
            System.out.println("IMPOSSIBLE");
        }
    }
} 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();

        if((A-B)/2==(double)(A-B)/2){
            if(A>=B){
                System.out.println(A-(A-B)/2);
            }
            if(A<B){
                System.out.println(B-(B-A)/2);
            }
        }
        else{
            System.out.println("IMPOSSIBLE");
        }
    }
} Main public static void main(String[] args){
        Scanner sc = new Scanner(System.in);

        int A = sc.nextInt();
        int B = sc.nextInt();

        if((A-B)/2==(double)(A-B)/2){
            if(A>=B){
                System.out.println(A-(A-B)/2);
            }
            if(A<B){
                System.out.println(B-(B-A)/2);
            }
        }
        else{
            System.out.println("IMPOSSIBLE");
        }
    } main {
        Scanner sc = new Scanner(System.in);

        int A = sc.nextInt();
        int B = sc.nextInt();

        if((A-B)/2==(double)(A-B)/2){
            if(A>=B){
                System.out.println(A-(A-B)/2);
            }
            if(A<B){
                System.out.println(B-(B-A)/2);
            }
        }
        else{
            System.out.println("IMPOSSIBLE");
        }
    } 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 if((A-B)/2==(double)(A-B)/2){
            if(A>=B){
                System.out.println(A-(A-B)/2);
            }
            if(A<B){
                System.out.println(B-(B-A)/2);
            }
        }
        else{
            System.out.println("IMPOSSIBLE");
        } (A-B)/2==(double)(A-B)/2 (A-B)/2 (A-B) A B 2 (double)(A-B)/2 (double)(A-B) A B 2 {
            if(A>=B){
                System.out.println(A-(A-B)/2);
            }
            if(A<B){
                System.out.println(B-(B-A)/2);
            }
        } if(A>=B){
                System.out.println(A-(A-B)/2);
            } A>=B A B {
                System.out.println(A-(A-B)/2);
            } System.out.println(A-(A-B)/2) System.out.println System.out System System.out A-(A-B)/2 A (A-B)/2 (A-B) A B 2 if(A<B){
                System.out.println(B-(B-A)/2);
            } A<B A B {
                System.out.println(B-(B-A)/2);
            } System.out.println(B-(B-A)/2) System.out.println System.out System System.out B-(B-A)/2 B (B-A)/2 (B-A) B A 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 A = sc.nextInt();
        int B = sc.nextInt();

        if((A-B)/2==(double)(A-B)/2){
            if(A>=B){
                System.out.println(A-(A-B)/2);
            }
            if(A<B){
                System.out.println(B-(B-A)/2);
            }
        }
        else{
            System.out.println("IMPOSSIBLE");
        }
    }
} public class Main{
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);

        int A = sc.nextInt();
        int B = sc.nextInt();

        if((A-B)/2==(double)(A-B)/2){
            if(A>=B){
                System.out.println(A-(A-B)/2);
            }
            if(A<B){
                System.out.println(B-(B-A)/2);
            }
        }
        else{
            System.out.println("IMPOSSIBLE");
        }
    }
} 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();

        if((A-B)/2==(double)(A-B)/2){
            if(A>=B){
                System.out.println(A-(A-B)/2);
            }
            if(A<B){
                System.out.println(B-(B-A)/2);
            }
        }
        else{
            System.out.println("IMPOSSIBLE");
        }
    }
}