#include <iostream>

using namespace std;

typedef long long int int64_t;

int64_t gcd(int64_t a, int64_t b) {
	while (1) {
		if (a < b) {
			b = b % a;
			if (b == 0) return a;
		} else if (b < a) {
			a = a % b;
			if (a == 0) return b;
		} else if (a == b)
			return a;
	}
}

int main()
{
	int64_t a, b;
	cin >> a >> b;
	for (int64_t d = b-a; d > 0; d--) {
		for (int64_t i = a; i + d <= b; i++) {
			if (gcd(i, i+d) == 1) {
				cout << d+1 << endl;
				return 0;
			}
		}
	}
	cout << -1 << endl;
	return 0;
}