import java.io.*;
import java.util.*;

public class Main {
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner();
        int a = sc.nextInt();
        int b = sc.nextInt();
        if (getGCD(a, b) > 1) {
            System.out.println(-1);
            return;
        }
        boolean[] enables = new boolean[a * b * 2 + 1];
        for (int i = 0; i <= b; i++) {
            for (int j = 0; j <= a; j++) {
                enables[i * a + j * b] = true;
            }
        }
        int count = 0;
        for (int i = 0; i < a * b; i++) {
            if (!enables[i]) {
                count++;
            }
        }
        System.out.println(count);
    }
    
    static int getGCD(int x, int y) {
        if (x % y == 0) {
            return y;
        } else {
            return getGCD(y, x % y);
        }
    }
}
class Scanner {
    BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
    StringTokenizer st = new StringTokenizer("");
    
    public Scanner() throws Exception {
        
    }
    
    public int nextInt() throws Exception {
        return Integer.parseInt(next());
    }
    
    public long nextLong() throws Exception {
        return Long.parseLong(next());
    }
    
    public String next() throws Exception {
        if (!st.hasMoreTokens()) {
            st = new StringTokenizer(br.readLine());
        }
        return st.nextToken();
    }
}