import java.util.Scanner;

public class Main {
	public static void main(String[] args) throws Exception {
		Scanner sc = new Scanner(System.in);
		int a = sc.nextInt();
		int b = sc.nextInt();
		sc.close();

		int c = Math.min(a, b);
		int d = Math.max(a, b);
		int end = a * b + d;
		int ans = 0;
		int cnt = 0;
		int i = 1;
		for ( ; i < end; i++) {
			if (can(i, c, d)) {
				cnt++;
			} else {
				ans++;
				cnt = 0;
			}
			if (cnt > d) {
				break;
			}
		}
		if (i == end) {
			System.out.println(-1);
		} else {
			System.out.println(ans);
		}
	}

	static boolean can(int x, int c, int d) {
		int end = x / d;
		for (int i = 0; i <= end; i++) {
			int r = x - (d * i);
			if (r % c == 0) {
				return true;
			}
		}
		return false;
	}
}