#include <cstdio>

int gcd(int a, int b) {
	return b == 0 ? a : gcd(b, a % b);
}

int		main(void)
{
	int		a, b;
	int		n, m;
	int		maxi, dif;

	scanf("%d%d", &a, &b);
	if (a == 1)
		printf("%d\n", b);
	else
	{
		n = b;
		m = a;
		while (gcd(n, m) != 1 && m < n)
			m++;
		if (m == n)
		{
			printf("-1\n");
			return (0);
		}
		maxi = n - m + 1;
		dif = m - a + 1;
		while (b - n + 1 < dif)
		{
			n--;
			m = a;
			while (gcd(n, m) != 1 && m < n)
				m++;
			if (n - m + 1 > maxi)
				maxi = n - m + 1;
		}
		printf ("%d\n", maxi);
	}
	return (0);
}