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 (gcd(a, b) != 1) {
		    System.out.println(-1);
		    return;
		}
		int max = a * b;
		boolean[] check = new boolean[max + 1];
		for (int i = 0; i * a <= max; i++) {
		    for (int j = 0; i * a + j * b <= max; j++) {
		        check[i * a + j * b] = true;
		    }
		}
		int count = 0;
		for (int i = 1; i <= max; i++) {
		    if (!check[i]) {
		        count++;
		    }
		}
		System.out.println(count);
   }
   
   static int gcd(int x, int y) {
       if (x % y == 0) {
           return y;
       } else {
           return gcd(y, x % y);
       }
   }
}