#include <bits/stdc++.h>

using namespace std;

#define ll long long
#define pb push_back
#define mp make_pair
#define pii pair<int, int>
#define pll pair<ll, ll>
#define all(x) (x).begin(), (x).end()
#define fi first
#define se second

ll a, b;

int main() {
	cin.sync_with_stdio(false);

	// freopen("a.in", "r", stdin);
	// freopen("a.out", "w", stdout);

	cin >> a >> b;

	if (__gcd(a, b) == 1) {
		cout << b - a + 1;
		return 0;
	}

	if (a + 1 <= b && __gcd(a + 1, b) == 1) {
		cout << b - a;
		return 0;
	}

	if (a <= b - 1 && __gcd(a, b - 1) == 1) {
		cout << b - a;
		return 0;
	}

	if (a + 1 <= b - 1 && __gcd(a + 1, b - 1) == 1) {
		cout << b - a - 1;
		return 0;
	}

	cout << -1;

	return 0;
}