

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.StringTokenizer;

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


import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.StringTokenizer;

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